Idle action thread

Right now, for each jail, there are 2 threads, one for the filter, and one for action.
If the filter backend is set to pyinotify (the default), then this thread has no active loop, which is great.

This work tries to do achieve the same goal for the action thread.
It use an unlimited sleep, using select, to be woken up when needed with a pipe write.
If there is something to unban, then the action thread sleep until that time, with a minimum of 1 second.

Discussion's opened. Critics are welcomed.