Implement decreasing health based on observing communication between
HAProxy and servers.
Changes in this version 2:
- documentation
- close race between a started check and health analysis event
- don't force fastinter if it is not set
- better names for options
- layer4 support
Changes in this version 3:
- add stats
- port to the current 1.4 tree
There are a few remaining max values that need to move to counters.
Also, the counters are more often used than some config information,
so get them closer to the other useful struct members for better cache
efficiency.
