GSI parallel compaction

Currently, GSI is limited to performing compaction on one index per node at a time. In the case that there are multiple indexes on one node, this can lead to disk use constantly growing as by the time one index has been compacted, the others have grown by a greater amount than was compacted. This is the case even when index traffic is within GSI's limitations.

This should be less of an issue with circular compaction in later versions, but still may be a useful feature as full compaction will be required from time to time.

An important caveat to note is the impact on diskIO of multiple compactors, see MB-18426.