Get rid of some direct uses of config in jvm_compile.

These are now replaced with registered options.

Got rid of the javac_args config key, as it was basically the same as
--args, except that pants added the -C escapes for you. This was confusing and
inconsistent anyway.

Created --source and --target options, and removed a random reference
to the old way of specifying those from test_rcfile.py, to avoid confusion
(even though the test still passed).

Testing Done:
All relevant unit and integration tests pass.

CI baking.

Reviewed at https://rbcommons.com/s/twitter/r/1357/
Remove some old-options cruft in tests.

Testing Done:
Unit tests pass.

Reviewed at https://rbcommons.com/s/twitter/r/1365/
Fix a bug related to adding sibling resource bases.

This bug was introduced in 3f5df746, a year ago:

git diff 3f5df746^..3f5df746 -- src/python/twitter/pants/tasks/jvm_task.py

Which leads me to believe that this code isn't necessary. Reviewers, please
chime in on whether we still need this sibling resource directory functionality
for sources, tests or both.

For example, do any pants.ini files at Twitter currently mention parallel_src_paths/parallel_test_paths?

It would be great to finally remove this code.

Reviewed at https://rbcommons.com/s/twitter/r/1367/
Remove execute_on_subproc and background subproc pool

Turns out we didn't get the parallelism we wanted with threads making blocking calls to subprocesses.

Re-using the subproc_map that is used in cache reads, and sending the whole call as a single piece of Work,
gets parallel compression and uploads, though at the expense of slightly less granular reporting. It also
turns out that separate process pools for aren't that useful, and slows down startup/shutdown.