Implement a graceful shutdown #1748
Description
Currently Symbolicator does not implement a graceful shutdown for SIGTERM or Ctrl+C, which is especially noticeable in a docker container when the service runs as pid 1 and hangs until it is killed, due to not reacting to SIGTERM.
Graceful shutdown can be implemented using the elegant-departure crate, as used in the objectstore and taskworkers, or manually like it is currently done in Relay.