Endpoint hints manager keeps a list of segments to replay. New segments
are appended to it lazily - only when a hint flush occurs (hints
commitlog instance is re-created) and the list is empty. Because of
that, this list cannot be currently used to tell how many segments are
on disk.
This commit allows to trigger hints flush and forcefully update the list
of segments to replay. In later commits, a mechanism will be implemented
which will allow to wait until a given number of hint segments is
replayed. Triggering a hints flush with segment list update will allow
us to properly synchronize and determine up to which segment we need to
wait.
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