[ASTERIXDB-2552][RT] Implement micro external sort for subplans - user model changes: no - storage format changes: no - interface changes: no
Details: This patch is to change the in memory sort used in subplans to be a micro external sort to avoid out of memory error.
- added reset() to the runs merger to allow reusing the runs merger. - renamed "InMemoryStableSortPOperator" to "MicroStableSortPOperator". - changed the tag from "IN_MEMORY_STABLE_SORT" to "MICRO_STABLE_SORT". - added test cases.