In elastic#28350, we fixed an endless flushing loop which may happen on 
replicas by tightening the relation between the flush action and the
periodically flush condition.

1. The periodically flush condition is enabled only if it is disabled 
after a flush.

2. If the periodically flush condition is enabled then a flush will
actually happen regardless of Lucene state.

(1) and (2) guarantee that a flushing loop will be terminated. Sadly, 
the condition 1 can be violated in edge cases as we used two different
algorithms to evaluate the current and future uncommitted translog size.

- We use method `uncommittedSizeInBytes` to calculate current 
  uncommitted size. It is the sum of translogs whose generation at least
the minGen (determined by a given seqno). We pick a continuous range of
translogs since the minGen to evaluate the current uncommitted size.

- We use method `sizeOfGensAboveSeqNoInBytes` to calculate the future 
  uncommitted size. It is the sum of translogs whose maxSeqNo at least
the given seqNo. Here we don't pick a range but select translog one by
one.

Suppose we have 3 translogs `gen1={#1,#2}, gen2={}, gen3={#3} and 
seqno=#1`, `uncommittedSizeInBytes` is the sum of gen1, gen2, and gen3
while `sizeOfGensAboveSeqNoInBytes` is the sum of gen1 and gen3. Gen2 is
excluded because its maxSeqno is still -1.

This commit removes both `sizeOfGensAboveSeqNoInBytes` and 
`uncommittedSizeInBytes` methods, then enforces an engine to use only
`sizeInBytesByMinGen` method to evaluate the periodically flush condition.

Closes elastic#29097
Relates #elastic#28350