FollowingEngine shouldn't fill history gaps upon promotion and recovery

[bleskes]

The following engine doesn't own the history but rather follows a remote one. As such, it can not close gaps on its own.

/cc @martijnvg @jasontedor

[elasticmachine]

Pinging @elastic/es-distributed

[martijnvg]

Gaps are closed in: Engine#fillSeqNoGaps(...)

I talked about this with @bleskes and he said the easiest way to achieve this is by making the fillSeqNoGaps(...) method a noop in the FollowingEngine implementation. Besides the unit test on the engine level, it would be good to have a test that simulates the need for this.

[martijnvg]

Implemented via #31751