Fix API to make the reverse requirement obvious

I fear a bit that we may accidentally reuse compare_epochs/2 in the
future without properly reversing the lists. This changes
compare_epochs/2 to compare_rev_epochs/2 and adds a new compare_epochs/2
that reverse its inputs before passing to compare_rev_epochs/2.
Obviously this required the reverse in find_common_seq_rpc/3.

BugzId: 27753