treat socket send errors as fatal for the device and call error callback
call asio recv functions only after our handlers are dispatched

unfortunately, asio seems to violate its own documentation and *does*
call handlers from async_* functions. restarting our receives after our
handlers have run mitigates this somewhat, but sync receive will still
be broken if packets arrive back to back.
add help text to hxb-net-autoconf
fix synchronous receive dropping packets under high load
Added Endpoints for Smartmeter
Added Wh as valid Unit
Merge branch 'kk-smartmeter' of ssh://github.com/mysmartgrid/hexabus into kk-smartmeter