abort-source: allow destroying abort source from a subscription
abort-source: allow destroying abort source from a subscription

A subscription may want to free an abort source that called it.
Make it possible by copying _subscriptions list into a local variable
before clearing it.

avikivity/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar