Improve thread safety docs for fields in ServerImpl

Split out of https://github.com/grpc/grpc-java/pull/2032#discussion_r70340687. `usingSharedExecutor` and `executor` aren't using `GuardedBy` semantics. It isn't clear to @carl-mastrangelo that the current code is valid. Improve the docs/code to correct/make the correctness more clear.

Start with ServerImpl and inspect the thread-safety handling for the usingSharedExecutor and executor fields. Read the discussion linked in the issue for the concern about GuardedBy semantics, then clarify the documentation or code so the existing correctness is unambiguous. Done means the synchronization expectations for both fields are clear.

Scritto dal modello di indicizzazione a partire dal testo della issue.