This adds an optional call_monitor_timeout parameter to the RPC client,
which if specified, will enable heartbeating of long-running calls by
the server. This enables the user to increase the regular timeout to
a much larger value, allowing calls to take a very long time, but
with heartbeating to indicate that they are still running on the server
side. If the server stops heartbeating, then the call_monitor_timeout
takes over and we fail with the usual MessagingTimeout instead of waiting
for the longer overall timeout to expire.
Change-Id: I60334aaf019f177a984583528b71d00859d31f84
b34ab8b1cc