Review ContextHandler locking

[sbordet]

Spawned by #5083.

Jetty version
9.4.x

Description
ContextHandler uses a mix of synchronized modifiers and synchronized (this) inside methods that is difficult to reconcile.

Furthermore, it has a setClassLoader() method that allow to arbitrarily change the _classLoader field but the rest of the code does not protect with a lock this field, nor it reads the field only once.
For example, loadClass(), handle(), addListener(), etc. reference the _classLoader field multiple times and the field may change between the references.

Also, handle() could have an improved logic.
In handle(), in the finally block, rather than checking on oldClassLoader the check should be done symmetrically on _classLoader like the code before the try does and like doStart() does.

Also, setAvailable() is locked, but isAvailable() is not. In general, _availability is not correctly locked.

[joakime]

I vote that .setClassLoader(ClassLoader) should not be allowed after the ContextHandler is started/running.

Changing the classloader after some classes have been loaded cannot lead to good things.