Batched notifications

[kpreid]

Right now, each individual change to a Space or other entity using listen::Notifier immediately iterates through the set of listeners and invokes all of them. This is probably rather cache-unfriendly and also contains redundant operations.

Try buffering notifications until the end of the overall operation (e.g. a transaction). Benchmark the results (e.g. modifying a Space with a listening raytracer). The buffer could be stored in the notifier itself, or it could be in a guard object which flushes the buffer on drop.

On the Listener side, change the signature to accept a slice of messages.

[kpreid]

As of 7b9d68b:

• Notifier & Listener process batches, and the new type Buffer allows creating batches.
• Space uses Buffer to batch notifications resulting from a set(), fill(), or transaction commit.