refactor: assert index is <= applied in compact_log instead of truncating

We used to automatically truncate the index in compact_log if the caller
requested to compact too many entries. This made it so that the
requirement of not compacting any un-applied entries was implicit in our
code base, which is not good as it allows for some bugs to creep in
(like the one we fix a couple commits ago).

Now this is changed and instead of silently adjusting the index of last
compacted entry, we just assert that it's no greater than the applied
index. As a consequence there's a minor improvement in do_raft_log_auto_compaction
function.