Re-enable JDK7 runtime compatibility for Javassist 3.x

pom.xml changes:
- lower compile source/target to 1.7 (from 1.8, introduced with Javassist 3.24.0)

It also adds a javadoc comment to CtClass#toClass() to recommend the users to use #toClass(Class) or #toClass(Lookup).

in other words, java.lang.invoke.MethodHandles.Lookup.

Add support for Java 11 NestHost and NestMembers attributes (JEP-181)

fixed JASSIST-267 (new )

Custom URL stream handler in ByteArrayClassPath

Clean code according to comments.

Otherwise a normal streamhandler lookup is made and if javassist is used early on server startup protses this can lead to a deadlock on rare cases on WebSpheare.
For example. A new StreamHandlerFactory is registered but not jet fully initialized. Now javassist is used at the same time but form another thread and it will be the first one to invoke the new StreamHandlerFactory and now it can deadlock with the other thread:

Snipets from my deadlock:
3XMTHREADINFO      "main" J9VMThread:0x000000000175EC00, omrthread_t:0x00007F4120008100, java/lang/Thread:0x00000000624290B8, state:B, prio=5
3XMTHREADBLOCK     Blocked on: org/osgi/util/tracker/ServiceTracker$Tracked@0x00000000627F0A90 Owned by: "other-thread" (J9VMThread:0x0000000001B26300, java/lang/Thread:0x00000000629F1458)
3XMHEAPALLOC             Heap bytes allocated since last GC cycle=0 (0x0)
3XMTHREADINFO3           Java callstack:
4XESTACKTRACE                at org/osgi/util/tracker/ServiceTracker.getServiceReferences(ServiceTracker.java:525)
4XESTACKTRACE                at org/eclipse/osgi/framework/internal/protocol/StreamHandlerFactory.createInternalURLStreamHandler(StreamHandlerFactory.java:162)
4XESTACKTRACE                at org/eclipse/osgi/framework/internal/protocol/StreamHandlerFactory.createURLStreamHandler(StreamHandlerFactory.java:118)
4XESTACKTRACE                at java/net/URL.getURLStreamHandler(URL.java:1154)
4XESTACKTRACE                at java/net/URL.<init>(URL.java:611(Compiled Code))
4XESTACKTRACE                at java/net/URL.<init>(URL.java:502(Compiled Code))
4XESTACKTRACE                at java/net/URL.<init>(URL.java:451)
4XESTACKTRACE                at javassist/ByteArrayClassPath.find(JRebel:92)
...
4XESTACKTRACE                at java/lang/ClassLoader.loadClassHelper(ClassLoader.java:924(Compiled Code))
5XESTACKTRACE                   (entered lock: org/eclipse/core/launcher/Main$StartupClassLoader@0x0000000062F3AA88, entry count: 1)

3XMTHREADINFO      "other-thread" J9VMThread:0x0000000001B26300, omrthread_t:0x00007F4120A5D060, java/lang/Thread:0x00000000629F1458, state:B, prio=5
3XMTHREADBLOCK     Blocked on: org/eclipse/core/launcher/Main$StartupClassLoader@0x0000000062F3AA88 Owned by: "main" (J9VMThread:0x000000000175EC00, java/lang/Thread:0x00000000624290B8)
3XMHEAPALLOC             Heap bytes allocated since last GC cycle=0 (0x0)
3XMTHREADINFO3           Java callstack:
4XESTACKTRACE                at org/osgi/util/tracker/AbstractTracked.size(AbstractTracked.java:356)
4XESTACKTRACE                at org/osgi/util/tracker/ServiceTracker.getServiceReferences(ServiceTracker.java:526)
5XESTACKTRACE                   (entered lock: org/osgi/util/tracker/ServiceTracker$Tracked@0x00000000627F0A90, entry count: 1)
4XESTACKTRACE                at org/eclipse/osgi/framework/internal/protocol/StreamHandlerFactory.createInternalURLStreamHandler(StreamHandlerFactory.java:162)
4XESTACKTRACE                at org/eclipse/osgi/framework/internal/protocol/StreamHandlerFactory.createURLStreamHandler(StreamHandlerFactory.java:118)
4XESTACKTRACE                at java/net/URL.getURLStreamHandler(URL.java:1154)
4XESTACKTRACE                at java/net/URL.<init>(URL.java:611(Compiled Code))
4XESTACKTRACE                at java/net/URL.<init>(URL.java:502(Compiled Code))
4XESTACKTRACE                at java/net/URL.<init>(URL.java:451)
4XESTACKTRACE                at java/net/URI.toURL(URI.java:1100)
4XESTACKTRACE                at com/ibm/jsse2/av.a(av.java:92)
4XESTACKTRACE                at com/ibm/jsse2/av.engineInit(av.java:7)
4XESTACKTRACE                at javax/net/ssl/TrustManagerFactory.init(TrustManagerFactory.java:25)

[#171] Allow running on JDK 8 and possibly on JDK 7 as well. Fixes #171

JASSIST-271 Allow running on Java 11 by falling back to ClassLoader.defineClass for Java > 10

[WIP] Fix leaking file handlers

Fixes to javassist.scopepool

Secure privileged

Unit Test: CtField.make referencing 'this'.

Fix for #152 inner class modifiers.

Applied fix for #152 "Wrong modifiers returned for nested classes" as suggested by @pietrobraione.
Includes unit tests for all access modifiers and static.

As per question raised in #161 passing 'this' reference for CtField value new object instance.
Also added assert for testing referncing 'this' for newly added field.