Implementing this Was harder than I first expected. The basic idea is as
follows: The master process puts each message arriving from a client that
isn't "canceltiles" into a (client-specific) queue. A separate thread that
pulls messages from the queue at its own pace and handles them as
before. Incoming "canceltiles" messages are handled specially, though: The
queue is emptied of "tile" messages.
The above sounds simple but there are several details that were a bit tricky
to get right.
