stop: allow double-calling Stop()

[tbg]

Stop() can be called multiple times and it's better to embrace that fact
than to try to fight it. See

cockroach/pkg/cli/start.go

Lines 794 to 809 in 89ce71e

	}
	stopper.Stop(drainCtx)
	}()
	// Don't return: we're shutting down gracefully.
	case <-log.FatalChan():
	// A fatal error has occurred. Stop everything (gracelessly) to
	// avoid serving incorrect data while the final log messages are
	// being written.
	// https://github.com/cockroachdb/cockroach/issues/23414
	// TODO(bdarnell): This could be more graceless, for example by
	// reaching into the server objects and closing all the
	// connections while they're in use. That would be more in line
	// with the expected effect of a log.Fatal.
	stopper.Stop(shutdownCtx)

Fixes #34572

Release note (bug fix): Fix a rare crash ("close of closed channel")
that could occur when shutting down a server.

[cockroach-teamcity]

This change is 

[petermattis]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @knz)

[tbg]

bors r=petermattis,knz

TFTRs

[craig]

Build succeeded

• GitHub CI (Cockroach)