[ASTERIXDB-2600][STO] Introduce ConcurrentMergePolicy - user model changes: yes. Add a new merge policy and make it as default - storage format changes: no. - interface changes: no.
Details: - Introduce ConcurrentMergePolicy that performs concurrent merges without the maximum component size. - Make this merge policy as the default merge policy in AsterixDB since the PrefixMergePolicy has made some wrong design decisions.