MB-19691: Address data race on vb_state::high_seqno

MB-19691: Address data race on vb_state::high_seqno

As identified by threadSanitizer (see below). Note that the read of
the disk highseqno is essentially pointless (it's only used in debug
logs), so just remove that to avoid the race.

WARNING: ThreadSanitizer: data race (pid=94198)
  Write of size 8 at 0x7d1000017fd8 by thread T14 (mutexes: write M15079):
    #0 CouchKVStore::saveDocs(unsigned short, unsigned long, _doc**, _docinfo**, unsigned long, KVStatsCtx&, unsigned long, unsigned long) ep-engine/src/couch-kvstore/couch-kvstore.cc:1804 (ep.so+0x000000325c3a)
    #1 CouchKVStore::commit2couchstore(Callback<KVStatsCtx>*, unsigned long, unsigned long) ep-engine/src/couch-kvstore/couch-kvstore.cc:1669 (ep.so+0x00000031f94f)
    #2 CouchKVStore::commit(Callback<KVStatsCtx>*, unsigned long, unsigned long) ep-engine/src/couch-kvstore/couch-kvstore.cc:1080 (ep.so+0x00000031f247)
    #3 EventuallyPersistentStore::flushVBucket(unsigned short) ep-engine/src/ep.cc:2790 (ep.so+0x0000000edbce)
    #4 Flusher::flushVB() ep-engine/src/flusher.cc:281 (ep.so+0x0000001ced95)
    #5 Flusher::step(GlobalTask*) ep-engine/src/flusher.cc:173 (ep.so+0x0000001cdce0)
    #6 FlusherTask::run() ep-engine/src/tasks.cc:45 (ep.so+0x000000251ffe)
   ...

  Previous read of size 8 at 0x7d1000017fd8 by thread T15:
    #0 CouchKVStore::getLastPersistedSeqno(unsigned short) ep-engine/src/couch-kvstore/couch-kvstore.cc:1091 (ep.so+0x00000031fe25)
    #1 DCPBackfill::run() ep-engine/src/dcp-stream.cc:149 (ep.so+0x00000028e590)

  Location is heap block of size 64 at 0x7d1000017fc0 allocated by thread T8:
    #0 operator new(unsigned long) <null> (engine_testapp+0x00000045c6ed)
    #1 CouchKVStore::snapshotVBucket(unsigned short, vbucket_state&, Callback<KVStatsCtx>*) ep-engine/src/couch-kvstore/couch-kvstore.cc:888 (ep.so+0x00000031b7f4)
    #2 EventuallyPersistentStore::snapshotVBuckets(Priority const&, unsigned short) ep-engine/src/ep.cc:970 (ep.so+0x0000000da799)
    #3 VBSnapshotTask::run() ep-engine/src/tasks.cc:49 (ep.so+0x0000002520be)

Change-Id: Iae368af8c2f0eceb67ce50d5e9358af56a75b6b2
Reviewed-on: http://review.couchbase.org/64211
Well-Formed: buildbot <build@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Will Gardner <will.gardner@couchbase.com>