Timer queue

The timer queue contains a list of FnOnce calls to execute at specific instants. What we need from this queue is:

• To give us the next expiry time
• To execute items from the front of the queue when time advances
• To add and delete items

A BinaryHeap might be good for this queue, except for the problem of deletions being O(N). So instead for the moment a BTreeMap is used. The map is partitioned to split off the items to execute when time advances. This should scale much better than a binary heap, especially considering deletions. However a BTreeMap generates a lot of code, so it is likely a combined N-ary heap tuned to cache line size and Vec to support deletion might perform better. So the underlying implementation will probably change at some point, once various scenarios have been benchmarked.

Apart from fixed timers, there are also "max" and "min" timers, that can be adjusted very cheaply (just a memory write), without having to add and delete timers from the timer queue most of the time.

For fixed timers, the BTreeMap maps a 64-bit key (32-bit wrapping expiry time + 31-bit unique value) to a boxed FnOnce. For min/max timers, the 64-bit key in the map consists of a 32-bit provisional expiry time and a 31-bit slot number. The actual current expiry time which is updated by the calls is kept in a separate array. When the timer expires, the current expiry time is checked, and another timer added back in if necessary.