db/commitlog: Implement skip in terms of input buffer skipping
db/commitlog: Implement skip in terms of input buffer skipping

This simplifies the code and allows to get rid of the overload of
advance() taking a temporary_buffer.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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