in_tail: Prevent the thread-unsafe method from being called concurrently

Essentially there is a concurrency bug in the implementation of `in_tail`.

While `IOHandler` is not designed to be thread-safe, its method `on_notify`
can be called by multiple threads concurrently. In particular, consider the
following situation:

 1. TailInput calls `on_notify` on detach/shutdown (main thread)
 2. In the same time, StatWatcher triggers `on_notify` on filesystem events
    (eventloop thread)

This patch fixes this issue by introducing a mutex, which prevent multiple
threads from entering into the critical session simultaneously.

lib/fluent/plugin/in_tail.rb

@@ -678,10 +678,15 @@ def initialize(watcher, &receive_lines)
           @iobuf = ''.force_encoding('ASCII-8BIT')
           @lines = []
           @io = nil
+          @notify_mutex = Mutex.new
           @watcher.log.info "following tail of #{@watcher.path}"
         end
 
         def on_notify
+          @notify_mutex.synchronize { handle_notify }
+        end
+
+        def handle_notify
           with_io do |io|
             begin
               read_more = false