wal: separate checkpoint and flush paths

Currently, wal_checkpoint() is used for two purposes. First, to make a
checkpoint (rotate = true). Second, to flush all pending WAL requests
(rotate = false). Since checkpointing has to fail if cascading rollback
is in progress so does flushing. This is confusing. Let's separate the
two paths.

While we are at it, let's also rewrite WAL checkpointing using cbus_call
instead of cpipe_push as it's a more convenient way of exchanging simple
two-hop messages between two threads.