Haproxyadmin

* Make sure we clear out possible previous errors
* Remove unnecessary keyword argument

* Implement a proper retry logic for socket failures

* Improve the way we internally use values for metrics

* Remove unnecessary filtering of empty values
* Fix broken design in converter function
* fix type in README
* cosmetic fix in doc string
* extend the support of error strings returned by haproxy
* add items in the TODO list
* mention from which socket file we don't get any data

* 9fbb459 didn't fix regression from dcc5173e31deac
* better handling of error when we connect to socket
* fix a regression introduced with dcc5173e31deac

* update TODO
* fix a regression introduced with dcc5173e31deac
* add support for sending commands to haproxy
* simplify the way we send commands to socket
* add support for keyword arguments in cmd\_across\_all\_procs()
* fix (once again) format issues in TODO.rst
* fix format issues in TODO.rst
* add some ordering in our TODO items