Revisit the mutex and atomics usage for BleveDest and BleveDestPartition within pindex_bleve

We've made changes that introduces both atomics and mutexes within the 2 structs - BleveDest and BleveDestPartition within cbft/pindex_bleve.go with these changes ..

• http://review.couchbase.org/c/cbft/+/149527
• http://review.couchbase.org/c/cbgt/+/149628

It's desirable to not use atomic.Value which makes the code less type-safe - so we should think of an alternative for ..

• lastUUID within BleveDestPartition
• partitions within BleveDest

Also, the use of atomics and mutexes together makes the code more prone to error during development - so some work needed to improve things there as well.