[MAJOR] process_session: rely only on buffer flags

[MAJOR] process_session: rely only on buffer flags

Instead of calling all functions in a loop, process_session now
calls them according to buffer flags changes. This ensures that
we almost never call functions for nothing. The flags settings
are still quite coarse, but the number of average functions
calls per session has dropped from 31 to 18 (the calls to
process_srv dropped from 13 to 7 and the calls to process_cli
dropped from 13 to 8).

This could still be improved by memorizing which flags each
function uses, but that would add a level of complexity which
is not desirable and maybe even not worth the small gain.