Multiplexer: Take the maximum number of events as a hint

Multiplexer: Take the maximum number of events as a hint

This allows indicating the maximum number of events we want to process
in a single run, which is usually bounded by the number of file descriptors
we are planning on watching.
The default is still 1024 events, but this change makes it possible to
allocate a smaller vector of events to reduce the memory usage when we
know we are going to need to process so many events in a single run.