Fixes a potential thread leak in the gRPC runtime. (#3757)

* Increase the number of request slots for CleanupGraph.

This WorkerService RPC method can receive a large number of
calls (especially at shared workers, such as parameter servers), leading
to starvation. In some cases, this can cause a thread to leak in the
master, as described in #3470. We currently register only a single
pending request for this RPC in each worker; this change increases the
count to 100 (to match RunGraph).

Fixes #3470.

* Avoid creating a thread to cleanup a step in gRPC session.

This change modifies the implementation of the distributed runtime's
Session::Run() implementation, so that it does not spawn a thread to
conduct (asynchronous) cleanup work.

* Remove accidentally-added dead code