Simplify failure detection and merge timeout configuration

FD/FD_ALL/FD_ALL2/FD_SOCK javadoc doesn't give any guidance as to how long it would take to detect a leaving member. MERGE2/MERGE3 javadoc also doesn't say how much it would take to detect that the network has healed.

For an example of how misleading the current settings can be, I have seen MERGE3 take more than 20s to merge two partitions with min_interval=1000 and max_interval=5000. FD also detects a leaver after timeout * max_tries in the best case, and twice that if 2 consecutive nodes (in the members list) leave at the same time.

The maximum time it takes to detect a leaver is of particular interest to Infinispan users, because Infinispan is supposed to protect against nodes leaving. But if the users don't configure a high enough RPC timeout in Infinispan, we don't get to detect the node leaving.

Ideally, the user should be able to specify a maximum detection time, and the protocol should adjust the existing settings to meet that (most of the time).