ShutdownHook: run on bundle deactivation if in OSGi

ShutdownHook: run on bundle deactivation if in OSGi

Running as a JVM shutdown hook is far too late in an OSGi framework; by
the time the JVM shuts down, the OSGi framework will normally already
have deactivated and unloaded bundles, and thus the JGit cleanup code
may try to work with unloaded classes for which there will be no
classloader anymore.

When JGit is used in an OSGi framework, the cleanups must run on bundle
deactivation, not on JVM shut down.

Add a declarative OSGi service CleanupService. This is a normal Java
class that has no dependencies on any OSGi bundle or interface, but
that is declared in the MANIFEST.MF and in an OSGi Service XML as an
OSGi immediate component. Set the bundle activation policy to "lazy".
(A declarative service is used instead of a bundle activator because the
latter would need to implement the OSGi interface BundleActivator, but
JGit should not have dependencies on OSGi.)

When JGit runs in an OSGi framework, the framework will create an
instance of CleanupService through the no-args constructor when (and
before) the first class from this bundle is loaded. This instance thus
knows that it is operating in OSGi, and will run the ShutdownHook when
the bundle is deactivated: bundle deactivation will deactivate the
CleanupService instance.

When JGit does not run in an OSGi framework, the OSGi service
declaration will be ignored, and there will be no already existing
CleanupService instance. We create one lazily, which thus knows that
it is not operating in OSGi, and which will use a JVM shutdown hook to
run the ShutdownHook.

This also reverts commit e6d83d61eade6dee223757d149a4df9650752a55.

Bug: jgit-36
Change-Id: I9c621b0707453c087f638974312ea1bf8ec30c31
Signed-off-by: Thomas Wolf <twolf@apache.org>