Refactor timerfd.cc

In his review of timerfd.cc, Avi asked that I simplify the implementation
by having a single "timerfd" object (instead of two I had - timerfd_file
and timerfd_object), and by using a single mutex instead of the complex
combination of mutexes and atomic variable.

This new version indeed does this. It should be easier to understand this
code, and it is 30 lines shorter.

The performance of this code is slightly inferior to the previous one -
in particular poll() now locks and unlocks a mutex - but this should be
negligible in practice.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>