Add a ten-second timeout to the DeleteWorkerSession call.

Previously, `MasterSession::Close()` did not block on the cleanup RPCs
to the individual workers, leading to deployments where the remote
workers might be shut down (e.g. by an external mechanism) before the
session was closed. In order to switch over to using
DeleteWorkerSession for all sessions, and preserve backwards
compatibility, we need to permit this behavior. Therefore, this CL
adds a 10-second timeout on the requests to workers, and logs an error
if the request does not succeed in that time period.

PiperOrigin-RevId: 193441618