graceful restart isn't graceful when more than one socket is configured
We terminate gitaly on the first failure from a listening socket, but on a graceful restart, we must wait for all the socket to properly terminate active connections.
Without a fix, gitaly graceful restart will be really graceful only on configuration with a single listener (Unix, TCP, or TLS).
note: we use 2 listeners on gitlab.com
!1190 (merged) is a proposed fix, and !1192 (closed) is a proposed refactoring (including the fix)