siso: complete upstream Execute streams cleanly in the proxy

The proxy returned from its Execute and WaitExecution handlers right
after forwarding the done operation, abandoning the upstream stream.
gRPC cancels a stream that its handler stops reading, so the backend
recorded every Execute as canceled even though it succeeded. Anything
that counts errors upstream sees the whole build as failing RPCs.

Read the trailing EOF the backend sends after the done operation before
returning, so the stream completes cleanly.

Change-Id: I32b85bda5f18de4769fd0c891807f39c084a82e2
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8114149
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Philipp Wollermann <philwo@google.com>