MINOR: stream: Add flags to identify the stream tansaction when allocated

MINOR: stream: Add flags to identify the stream tansaction when allocated

To be able to deal with different types of transaction for a stream, new
stream flags was added to know the transaction type when allocated. For now
only HTTP transactions can be allocated, so only SF_TXN_HTTP was
introduced. The mask SF_TXN_MASK must be used to get the transaction type.

The transaction type is set when it is allocated and removed when it is
destroyed.