b/31228845: wait for query killer to exit (#2019)
b/31228845: wait for query killer to exit (#2019)

We found a race condition where the query killer goroutine may
get delay to the extent that the original routine returns first
and close the connection (due to an error). If this happens, then
the killer ends up trying to kill a closed connection which results
in a seg fault.

The new code waits for the killer routine to exit before returning.
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