rt: fix shutdown deadlock in threaded scheduler

[carllerche]

Previously, when the threaded scheduler was in the shutdown process, it
would hold a lock while dropping in-flight tasks. If those tasks
included a drop handler that attempted to wake a second task, the wake
operation would attempt to acquire a lock held by the scheduler. This
results in a deadlock.

Dropping the lock before dropping tasks resolves the problem.

Fixes #2046