Change merged-segment-removing behavior in RealtimePlumber.

Old: Merged segments are deleted upon successful push, or on IOException.
New: Merged segments are deleted on any Exception, but NOT successful push.

Deleting a merged segment means that the next merge-and-push run will try to
push it again. So we want that to happen if there was any sort of Exception.
We *don't* want it to happen if the merge-and-push was successful, since in
that case, we are just waiting for historical nodes to load the segment. It
might take a while, but there's no reason to re-push while waiting.
?