*Monitor sproxyd backend servers and validate their configuration

Try to connect to each Sproxyd server in the `sproxyd_host` list
every second and parse the configuration read from /proxy/chord_path/.conf.
If something wrong happens for a certain time (actual implementation
based on the Accrual Failure Detector), the incriminated Sproxyd server
is removed from the list of Sproxyd servers the Swift object server talks
to.

Also, slightly change the implementation of the Accrual FD.

Closes #17
Closes #20