Use Thread::Mutex to resolve race condition

[okkez]

The problematic sequence:

1. Add last line to @buffer
2. Call flush_buffer in flush_timeout_buffer (via periodical timer)
3. flush_buffer

Sequence 2 and 3 may race.

[jdef]

has this race otherwise been resolved?

[okkez]

This is very rare case. I cannot reproduce this race condition in my environment.