future-util: reduce allocations in repeat
future-util: reduce allocations in repeat

Allocate a single reusable task for repeat and call into it
any time we have to defer.

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