txn_limbo: fix promote term filtering

txn_limbo_process() used to filter out promote requests whose term was
equal to the greatest term seen. This wasn't correct for PROMOTE entries
with term 1.

Such entries appear after box.ctl.promote() is issued on an instance
with disabled elections. Every PROMOTE entry from such an instance has
term 1, but should still be applied. Fix this in the patch.

Also, when an outdated PROMOTE entry with term smaller than already
applied from some replica arrived, it wasn't filtered at all. Such a
situation shouldn't be possible, but fix it as well.

Part-of #6034