Spinoff from here:
The addIndexesNoOptimize method first merges eligible segments
according to the MergePolicy, and then copies over one by one any
remaining "external" segments.
That copy can possibly (rather rarely) result in new merges becoming
eligible because its size can change if the index being added was
created with autoCommit=false.
However, we fail to then invoke the MergeScheduler to run these
merges. As a result, in close, where we wait until all running and
pending merges complete, we will never return.
The fix is simple: invoke the merge scheduler inside
copyExternalSegments() if any segments were copied. I also added
defensive invocation of the merge scheduler during close, just in case
other code paths could allow for a merge to be added to the pending
queue but not scheduled.