[ENGINE] Add current translog ID to commit meta before closing

Today we simply close the IW when we have to open a new one if ie.
we change merge settings. Yet, the API is deprecated and under the hood
it commits the IW. For safety we should add the current translog ID and
use rollback instead to not wait for merges.
Asking for my own information here, but would it be more efficient to use `new HashMap<String, String>(1)` because we always know the capacity for the HashMap? The default constructor has an `initialCapacity` of 16, does it save much to manually lower it?