DoH: Simplify http_do_bio()

DoH: Simplify http_do_bio()

This commit significantly simplifies the code flow in the
http_do_bio() function, which is responsible for processing incoming
and outgoing HTTP/2 data. It seems that the way it was structured
before was indirectly caused by the presence of the missing callback
calls bug, fixed in 8b8f4d500d9c1d41d95d34a79c8935823978114c.

The change introduced by this commit is known to remove a bottleneck
and allows reproducible and measurable performance improvement for
long runs (>= 1h) of "stress:long:rpz:doh+udp:linux:*" tests.

Additionally, it fixes a similar issue with potentially missing send
callback calls processing and hardens the code against use-after-free
errors related to the session object (they can potentially occur).

(cherry picked from commit 0956fb9b9ec4d07f1b744a829dacfe7a1251a58e)