merge Client and ClientKeep

[tharvik]

Troughout onet/cothority, there is a separation made between client using keep-alive or not. I don't think that's necessary to have a specific interface to the keep-alive one. Also, is there a realistic use case where someone doesn't want to keep-alive? While I'm at it, timeout is also used around but rarely modified. Something like having an single configuration for client, with sane defaults on nil-values would fix it.