This change depends on the recently introduced mossStore Stats() API
in github.com/couchbase/moss 564bdbc0 commit. So, gvt for moss has
been updated as part of this change.
Most of the change involves propagating the mossStore instance (the
statsFunc callback) so that it's accessible to the KVStore.Stats()
method.
See also: http://review.couchbase.org/#/c/67524/
Previously, the PrefixIterator() for moss was implemented by comparing
the prefix bytes on every Next().
With this optimization, the next larger endKeyExclusive is computed at
the iterator's initialization, which allows us to avoid all those
prefix comparisons.
The moss RegistryCollectionOptions allows applications to register
moss-related callback API functions and other advanced feature usage
at process initialization time.
For example, this could be used for moss's OnError(), OnEvent() and
logging callback options.
Performance optimization. Before this change, by using Merge()
instead of AllocMerge(), moss's internal batch buf's would be
wastefully, dramatically grown during append()'s to a mis-sized buf.
