SQUASH "msgr: enable fast_dispatch on local connections"

Move local_delivery into a separate thread from the sender's. fast_dispatch
makes this entirely necessary. Set up the thread with a list of messages
and proceed as you'd expect from that.

This patch makes "msgr: drop SimpleMessenger::lock while doing local_dispatch"
pointless as well.

Signed-off-by: Greg Farnum <greg@inktank.com>