commitlog: reduce dependencies on db/config.hh
commitlog: reduce dependencies on db/config.hh

Instead of accessing extensions via config, access it via
database::extensions(). This reduces recompilations when configuration
is extended.

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