stream: don't use ssn timestamp flag in stream

The STREAMTCP_FLAG_TIMESTAMP flag is a ssn flag, however it was used in
the stream flag field. As it has the same value as
STREAMTCP_STREAM_FLAG_DEPTH_REACHED it's possible that stream reassembly
got confused by the timestamp.
stream: zero ts is a per stream flag

Ssn flag STREAMTCP_FLAG_ZERO_TIMESTAMP was used in stream only. Due to
it's value it did not conflict with a real stream flag. Renamed it to
STREAMTCP_STREAM_FLAG_ZERO_TIMESTAMP.
stream: remove unused 'pause' feature
stream: intro function for SYN/ACK state update

As the TCP SSN state can be updated from several points in the state
machine on accepting a SYN/ACK, move the update logic into a separate
function.