MB-9161 Reset engine_specific to NULL when chk_persistence fails

If a checkpoint_persistence request fails due to the timeout,
we should then reset the engine_specific value to NULL, so that
the next checkpoint persistence request should work correctly.

Change-Id: Iab0ab3c41fe39dc7060c3d094ec04cb9cfadcb73
Reviewed-on: http://review.couchbase.org/30677
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Merge remote-tracking branch 'couchbase/2.5.0'

* couchbase/2.5.0:
  MB-9161 Reset engine_specific to NULL when chk_persistence fails

Change-Id: I2258a56912833af4207a577e48dd2a784e5990e8