htools: add confd server module

This contains a more-or-less complete implementation for the
server-side confd.

Note that most of the code is behaving identical to the Python code,
with a notable exception: the asyncore/inotify code was changed (since
Haskell doesn't have asyncore) to 3-thread system. While otherwise I
tried to keep the code flow identical, I found the original Python
code not easily to reason about, and as such I changed the
architecture in this regard.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>