net_updates: combine handlers, parametrized by update approach
net_updates: Txn module to handle RPC-like switch communication

This module exposes a send function that will return an Ivar.t that
will be filled with the response from the switch whe the controller
receives the response. The management of xids is hidden from the user.

Be sure to only use Txn.send with messages that will provoke the switch
to send a response. If you don't and you block on reading the returned
Ivar.t, then you'll be waiting until the switch disconnects.
net_updates: Use Txn for barriers, treat error and disconnect differently

Replace code for barrier requests with the Txn module. Txn will detect
switch disconnects, so it's no longer necessary to try and guess when
that happens using a timeout. Doing this also enables the code to
distinguish disconnect and error scenarios.