queue: add front()
queue: add front()

Allow accessing the front element without committing to popping it.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20201217124239.605858-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar