Use a ReadWriteLock rather than a single, global lock to memoize in dagger.

Use a ReadWriteLock rather than a single, global lock to memoize in dagger.

In server-side apps we have clocked Dagger's LRU cache locks as the third-most contended lock, and this permits much more parallelization.  While less crucial on Android, this is a performance killer on a high-contention server.
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