With backoff client, setting .timeout property on it doesn't take any
effect. It means that starting from Mitaka, we broke
quitting_rpc_timeout option.
Now, when the TERM signal is received, we reset the dict capturing
per-method timeouts; and we cap waiting times by the value of the
option. This significantly reduces time needed for the agent to
gracefully shut down.
Change-Id: I2d86ed7a6f337395bfcfdb0698ec685cf384f172
Related-Bug: #1663458