refactor: add drain() seam to threadHandler interface

[nicolas-grekas]

Third step of the split suggested in #2287: land the handler-interface
extension point that later handler types (background workers) need,
without introducing any new behaviour.

Each handler gains a drain() method, called by drainWorkerThreads
right before drainChan is closed. All current implementations
(regularThread, workerThread, inactiveThread, taskThread) are
no-ops, so observable behaviour is unchanged. A later handler that
needs to wake up a thread parked in a blocking C call (e.g. by closing
a stop pipe) plugs its signal in here without modifying
drainWorkerThreads again.

What

• phpthread.go — interface gains drain().
• threadregular.go / threadworker.go / threadinactive.go /
  threadtasks_test.go — empty drain() on each handler.
• worker.go — drainWorkerThreads calls thread.handler.drain()
  right before close(thread.drainChan).

Net change: +15 / -0.