learning-switch: Delay sending handshake until version negotiation is done.

learning-switch: Delay sending handshake until version negotiation is done.

The learning-switch implementation needs to know the OpenFlow version in
use to send the initial handshake messages (e.g. the feature request), but
the version is not always available at the time that the code currently
sends the handshake.  This can cause an assertion failure later when
ofputil_encode_flow_mod() checks the protocol, which will be 0 if the
version wasn't known.

This commit fixes the problem by introducing a state machine that sends the
handshake messages only after version negotiation has finished.

Reported-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>