Configurable shutdown delay

[stuarteberg]

When I send dvid a SIGTERM, it shuts down cleanly, which is nice. But it adds a deliberate 5-second delay:

2018/12/03 17:47:27 Stop signal captured: "terminated".  Shutting down...
 INFO No chunk handlers active. Proceeding...
 INFO Waiting 5 seconds for any HTTP requests to drain...
 INFO All 0 data instances shutdown.
 INFO Checking for any active cgo routines...
 INFO Halted 1 RPC servers.
2018/12/03 17:47:32  INFO Shutting down DVID core...
2018/12/03 17:47:33  INFO Logging system shutdown.
2018/12/03 17:47:33 Closing log file...

When writing unit tests for dvid clients, it may be necessary to quickly launch and shutdown dvid servers multiple times in a single testing session, but that 5-second delay makes the tests take longer than necessary.

It would be nice if that shutdown delay were configurable via dvid's toml file.