[MINOR] http: introduce a new synchronisation state : HTTP_MSG_DONE

[MINOR] http: introduce a new synchronisation state : HTTP_MSG_DONE

This state indicates that an HTTP message (request or response) is
complete. This will be used to know when we can re-initialize a
new transaction. Right now we only switch to it after the end of
headers if there is no data. When other analysers are implemented,
we can switch to this state too.

The condition to reuse a connection is when the response finishes
after the request. This will have to be checked when setting the
state.