conn: dont propogate context cancelled in prepare (apache#1368)

* conn: dont propogate context cancelled in prepare

If a callers context is cancelled and they won the race to prepare a
statement, dont stop the prepare and fail it with context failed error
as other callers may be waiting for us to finish.

Achieve this by replacing the waitgroup used to wait for prepared to
finish with a channel which is closed by the preparer. Do the preparing
in a goroutine and use context.Background in the exec call so that the
prepare will always finish. All callers now use the same flow to wait
for the result of the prepare waiting against their own context so they
can bail out individually.

fixes apache#1341

* ensure closing a connection kills inflight prepares