Replica should handle aborts for non-pending stored values during disk backfill [ETA 2019/7/12]

Deduplication in a disk snapshot can lead to replicas receiving an abort, but finds the item in the hashtable is not pending.

Consider the active receives the following ops for a given key

PRE1 CMT1 PRE2 ABRT2

The replica connects, backfills from disk, and will see

SET ABRT2

When processing the abort in VBucket::abort, ht.findForWrite(key) will find the stored value, but it will not be pending - it will be the value stored by the SET.

The exception will be thrown from around kv_engine/engines/ep/src/vbucket.cc:912

This should only be possible when receiving a disk snapshot, so it could be special cased.