raft: drop waiters with outdated terms
raft: drop waiters with outdated terms

Currently an entry is declared to be dropped only when an entry with
different term is committed with the same index, but that may create a
situation where, if no new entries are submitted for a long time, an
already dropped entry will not be noticed for a long time as well.

Consider the case where a client submits 10 entries on a leader A, but
before they get replicated the leadership moves to a node B. B will
commit a dummy entry which will be committed eventually and will release
one of the waiters on A, but if anything else is submitted to B 9 other
waiters will wait forever.

The way to solve that is to drop all waiters that wait for a term
smaller that one been committed. There is no chance they will be
committed any longer since terms in the log may only grow.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla