Slight refactoring of Server, use unmapSlave everywhere instead of accessing the corresponding map

Remove comment from HaIdGeneratorFactory
Fixes race on master between unmapSlave and channelDisconnected

It is possible for the slave to close and disconnect a channel before the master
 has had the opportunity to unmap it. In this case, it is possible for the
 channelDisconnected handler in Server to try and finishTransaction() on a channel
 that is actually not mapped to one, leading to distracting exceptions. By unmapping
 the slave before handling the request we ensure that this cannot happen.
 This is safe because the mapping is not used when handling the request,
 including when managing failure.