Implement the fast-delivery change.

Still not tested though. And I wonder if using IxSet is a bit
overkill...
Fixed a bug in the use of IxSet.

Exceptions inside STM are bad...

The fast-delivery change seems to have greatly improved the
latency. However, it's also generating a lot of duplicated ACKECHO and
ACK packets. Why?
Seems that IxSet is leaking.

Hmm.. Let's firstly check if this works tonight.
Then we may consider building a multi-index map by ourselves.