ACID: use a new write id for compaction's output instead of the visibility id

HIVE-20823 added support for running compaction within a transaction. To control the visibility of the output directory, it uses base_writeId_visibilityId, where visibilityId is the transaction id of the transaction that the compactor ran in. Perhaps we can keep using the base_writeId format, by allocating a new writeId for the compactor and creating the new base/delta with that.