MB-21539: Address raciness / intermittent timeout in test_mb16357

MB-21539: Address raciness / intermittent timeout in test_mb16357

This is a multithreaded test which attempts to run the compactor
concurrently with a DCP stream when a vBucket flips to
replica. However, there is no interlocking of the threads' startup,
which can result in the compaction running and completing before the
DCP thread reaches line 810 (where it busy-waits for
ep_pending_compactions != 0). As a result the test can get stuck at
that line and timeout.

Address this by introducing synchronisation between the two threads -
ensure that the compactor thread is ready and about to compact before
advancing the DCP thread.

Change-Id: Icc5abb3080a981823bb2161d083f0d866364b76e
Reviewed-on: http://review.couchbase.org/70085
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>