[mossScope] Add a compact option

Request from support:

Add a compact option to mossScope, or at least an internal settings option, so the customer wouldn't be aware of it, as one could potentially disrupt front end operations with this power. This API would invoke compaction only if the number of open handles to the store are zero (need to figure a way for this).

The reason behind this is so that support could use it whenever needed if the underlying store files have become super large.