reactor: don't become inefficient when max_task_backlog is exceeded
reactor: don't become inefficient when max_task_backlog is exceeded

When need_preempt() becomes true, task execution becomes inefficient:
all loops are broken into at least a task per loop, and even .then()
chains cannot execute their inline branch any more and are forced
to allocate and queue a task. Since we already have a backlog, this
makes it very hard to return to normal processing.

Fix by clearing the flag. The preemption tick will set it again, but
sufficiently far in the future.

Fixes #302.

Tests: unit (release)
Message-Id: <ab82b12073f4bbff255869849bab79b3cedf1e22@scylladb.com>

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