Fix all Javadoc warnings and fail on them

Fix all Javadoc warnings and fail on them

This fixes all the javadoc warnings, stops ignoring doclint 'missing'
category and fails the build on javadoc warnings for public and
protected classes and class members.

Since javadoc doesn't allow access specifiers when specifying doclint
configuration we cannot set `-Xdoclint:all,-missing/private`
hence there is no simple way to skip private elements from doclint.
Therefore we check javadoc using the Eclipse Java compiler
(which is used by default) and javadoc configuration in
`.settings/org.eclipse.jdt.core.prefs` files.
This allows more fine grained configuration.

We can reconsider this when javadoc starts supporting access specifiers
in the doclint configuration.

Below are detailled explanations for most modifications.

@inheritDoc
===========
doclint complains about explicits `{@inheritDoc}` when the parent does
not have any documentation. As far as I can tell, javadoc defaults to
inherit comments and should only be used when one wants to append extra
documentation from the parent. Given the parent has no documentation,
remove those usages which doclint complains about.

In some case I have moved up the documentation from the concrete class
up to the abstract class.

Remove `{@inheritDoc}` on overriden methods which don't add additional
documentation since javadoc defaults to inherit javadoc of overridden
methods.

@value to @link
===============
In PackConfig, DEFAULT_SEARCH_FOR_REUSE_TIMEOUT and similar are forged
from Integer.MAX_VALUE and are thus not considered constants (I guess
cause the value would depends on the platform). Replace it with a link
to `Integer.MAX_VALUE`.

In `StringUtils.toBoolean`, @value was used to refer to the
`stringValue` parameter. I have replaced it with `{@code stringValue}`.

{@link <url>} to <a>
====================
@link does not support being given an external URL. Replaces them with
HTML `<a>`.

@since: being invalid
=====================

org.eclipse.jgit/src/org/eclipse/jgit/util/Equality.java has an invalid
tag `@since: ` due to the extra `:`. Javadoc does not complain about it
with version 11.0.18+10 but does with 11.0.19.7. It is invalid
regardless.

invalid HTML syntax
===================

- javadoc doesn't allow <br/>, <p/> and </p> anymore, use <br> and <p>
instead
- replace <tt>code</tt> by {@code code}
- <table> tags don't allow summary attribute, specify caption as
<caption>caption</caption> to fix this

doclint visibility issue
========================

In the private abstract classes `BaseDirCacheEditor` and
`BasePackConnection` links to other methods in the abstract class are
inherited in the public subclasses but doclint gets confused and
considers them unreachable. The HTML documentation for the sub classes
shows the relative links in the sub classes, so it is all correct. It
must be a bug somewhere in javadoc.
Mute those warnings with: @SuppressWarnings("doclint:missing")

Misc
====
Replace `<` and `>` with HTML encoded entities (`&lt; and `&gt;`).
In `SshConstants` I went enclosing a serie of -> arrows in @literal.

Additional tags
===============
Configure maven-javad0c-plugin to allow the following additional tags
defined in https://openjdk.org/jeps/8068562:
- apiNote
- implSpec
- implNote

Missing javadoc
===============
Add missing @params and descriptions

Change-Id: I840056389aa59135cfb360da0d5e40463ce35bd0
Also-By: Matthias Sohn <matthias.sohn@sap.com>