Use r/w Mutex to control access to jvmtiEnv event information.

We were previously relying on the mutator_lock_ to provide
synchronization when accessing control information for some jvmti
events. This was not fully thread safe. To fix this problem we
instead use a ReaderWriterMutex to control access to this data.

Refactorings to allow all methods to have appropriate lock annotations
will follow in later CL (see bug).

Performance Impact:
    Tested performance by running main piece of test 993 10 times.
    Printing redirected to /dev/null.
    Total of ~270 breakpoints are executed.

    Before: 2.62 seconds / run
    After:  2.72 seconds / run

Test: ./test.py --host -j50
Test: ./art/tools/run-prebuilt-libjdwp-tests.sh --debug

Bug: 62821960
Bug: 67958496
Bug: 67962439

Change-Id: Ib2935c2e10bc2113e8430b49a9a7b76144e4235e
7 files changed