MEDIUM: http: add a prefetch function for ACL pattern fetch

MEDIUM: http: add a prefetch function for ACL pattern fetch

This function will be called by all ACL fetch functions. Right now all ACL
fetch functions have to perform the exact same tests to check whether data
are available. Also, only one of them is able to actually parse an HTTP
request.

Using the prefetch function, it will be possible to try to parse a request
on the fly and to avoid the fetch if some data are missing. This will
significantly reduce the amount of tests in all ACL fetch functions.