Update java docs to clarify thread safety of the client

[igorbernstein2]

The docs should make it clear that the client instances can be safely shared across threads. The methods that return non-threadsafe instances should be prominently documented as such on both the creator method and the class returned.

For example, BigtableDataClient#newBulkMutationBatcher() can return a Batcher instance that cannot be shared across threads. The docs for newBulkMutationBatcher() should explicitly state that the returned Batcher instance is not threadsafe, so cannot be used by more than one thread. And in addition, the Batcher interface should also explicitly state in its javadocs that the interface can only be used by a single thread.