First draft (3rd edition), ready for some review

Vnode replies always go via reply(), and reply() always uses unreliable
messaging.  (As opposed to the usual (and more reliable) send-and-pray
messaging.)

During handoff, all forwarding requests use unreliable vnode master
commands to avoid net_kernel blocking interference.
Review: fix return value of bang_unreliable/2
Add PULSE instrumentation to riak_core_send_msg.erl
Merge pull request #317 from basho/jrw-gh274-13backport

Backport of #299 to 1.3