Make stats gathering idempotent

[2opremio]

Leftover from #1683 , found while diagnosing #1795

[rade]

LGTM

[ekimekim]

There's a race where StartGatheringStats() could get called twice, and the second call happens before the line c.statsConn = conn is reached, so we'll start a second goroutine and have two things gathering stats.
But that's not a regression in this particular PR, so LGTM.

[2opremio]

There's a race where StartGatheringStats() could get called twice, and the second call happens before the line c.statsConn = conn is reached, so we'll start a second goroutine and have two things gathering stats.

Good catch, would you mind filing a bug for it?