Internal: an inactive shard is temporarily activated by triggered syn…

…ced flush

When a shard becomes in active we trigger a sync flush in order to speed up future recoveries. The sync flush causes a new translog generation to be made, which in turn confuses the IndexingMemoryController making it think that the shard is active. If no documents comes along in the next 5m, the shard is made inactive again , triggering a sync flush and so forth.

To avoid this, the IndexingMemoryController is changed to ignore empty translogs when checking if a shard became active. This comes with the price of potentially missing indexing operations which are followed by a flush. This is acceptable as if no more index operation come in, it's OK to leave the shard in active.

A new unit test is introduced and comparable integration tests are removed.

Closes elastic#13802