MINOR: http-ana: Simplify creation/destruction of HTTP transactions

MINOR: http-ana: Simplify creation/destruction of HTTP transactions

Now allocation and initialization of HTTP transactions are performed in a
unique function. Historically, there were two functions because the same TXN
was reset for K/A connections in the legacy HTTP mode. Now, in HTX, K/A
connections are handled at the mux level. A new stream, and thus a new TXN,
is created for each request. In addition, the function responsible to end
the TXN is now also reponsible to release it.

So, now, http_create_txn() and http_destroy_txn() must be used to create and
destroy an HTTP transaction.