If the translog flush threshold is too small (eg. smaller than the
translog header), we may repeatedly flush even there is no uncommitted
operation because the shouldFlush condition can still be true after
flushing. This is currently avoided by adding an extra guard against the
uncommitted operations. However, this extra guard makes the shouldFlush
complicated. This commit replaces that extra guard by a lower bound for
translog flush threshold. We keep the lower bound small for convenience
in testing.

Relates elastic#28350
Relates elastic#23606