identd: Rework lowestSocketId handling

identd: Rework lowestSocketId handling

Since we're appending only strictly increasing integers, the lowest
socketId is always at the front of the list (if that exists).

hasSocketsBelowId also had a off-by-one error. This had no effect on the
code since a socketId equal to the current _socketId cannot be in the
_waiting list.

Reorder processWaiting's handling of requests:
  - If the socketId is one the request might have been waiting for,
    check responseAvailable.
  - Otherwise, if there is at least one socket we need to wait for,
    continue waiting.
  - Otherwise, responseUnavailable.

The former order was:
  - If there are no sockets left from before this request was made,
    responseUnavailable.
  - If the socketId is one the request might have been waiting for,
    check responseAvailable.
  - Otherwise, continue waiting.

The change was necessary as the lowestSocketId no longer includes the
socketId that was just removed. In addition, the new order also matches
the if-else chain in IdentServer::respond.