net: tcp: Make the connect callback on success, not transmission

The net_context_connect() callback was being invoked synchronously
with the transmission of the SYN packet.  That's not very useful, as
it doesn't tell the user anything they can't already figure out from
the return code.  Move it to the receipt of the SYNACK instead, so the
app can know that it's time to start transmitting.  This matches the
Unix semantics more closely, where connect(2) is a blocking call that
wakes up only when the connection is live.

Change-Id: I11e3cca8572d51bee215274e82667e0917587a0f
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>