Right now replay is being issued with the standard seastar priority.
The rationale for that at the time is that it is an early event that
doesn't really share the disk with anybody.
That is largely untrue now that we start compactions on boot.
Compactions may fight for bandwidth with the commitlog, and with such
low priority the commitlog is guaranteed to lose.
Fixes#1856
Signed-off-by: Glauber Costa <glauber@scylladb.com>