[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.
Remove default node group. In this way, CREATE DATASET statement can adjust to dynamic cluster topology.
When we create a dataset: - if the node group name is not given, we create a new node group using all currently available nodes; - if the node group name is give, we use the given node group for the dataset.
When we drop a dataset: - if no other dataset depends on the node group of the dataset to be dropped, we also drop the node group.