timer: Do not require callback to be copyable
timer: Do not require callback to be copyable

Allows for unique_ptrs in lambda captures.
Message-Id: <525a369662bcb61cc6dffd60f3c3c85c4d653dc2@scylladb.com>

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