Run TailWatcher#on_notify before attaching triggers

[akihiro17]

related to #1229

This commit runs TailWatcher#on_notify before attaching stat_trigger and timer_trigger and prevents TailWatcher#on_notify from running in two different threads(main and event loop thread) at the same time.

This change fixes the can't modify string; temporarily locked error.

I think fluentd v0.12 does not raise the can't modify string; temporarily locked error because it runs stat_trigger and timer_trigger after TailInput#start finishes and thus TailWatcher#on_notify is not executed in two threads simultaneously.

# fluentd v0.12
# in_tail.rb
def start
  @loop = Coolio::Loop.new
  refresh_watchers

  @thread = Thread.new(&method(:run))
end

def run
  @loop.run
end

reproduction steps

same as described in #1229

1. Create large log files by using append.rb
2. Remove a pos file (rm tmp/logs/tail.pos)
3. Run fluentd
4. Run append.rb on another terminal

[repeatedly]

Thanks 👍
This seems the root cause.