BUG/MEDIUM: http: clear CF_READ_NOEXP when preparing a new transaction

Commit b1982e2 ("BUG/MEDIUM: http/session: disable client-side expiration
only after body") was tricky and caused an issue which was fixed by commit
0943757 ("BUG/MEDIUM: session: don't clear CF_READ_NOEXP if analysers are
not called"). But that's not enough, another issue was introduced and further
emphasized by last fix.

The issue is that the CF_READ_NOEXP flag needs to be cleared when waiting
for a new request over that connection, otherwise we cannot expire anymore
an idle connection waiting for a new request.

This explains the neverending keepalives reported by at least 3 different
persons since dev24. No backport is needed.