BUG/MINOR: stats: don't count tarpitted connections twice

When a connection is tarpitted, a denied req is counted once when the
action is applied, and then a failed req is counted when the tarpit
timeout expires. This is completely wrong as the tarpit is exactly
equivalent to a deny since it's a disguised deny.

So let's not increment the failed req anymore.

This fix may be backported to 1.4 which has the same issue.
CLEANUP: http: homogenize processing of denied req counter

The reqdeny/reqideny and reqtarpit/reqitarpit rules used to maintain
the stats counters themselves while http-request deny/tarpit and
rspdeny/rspideny used to centralize them at the point where the
error is processed.

Thus, let's do the same for reqdeny/reqtarpit so that the functions
which iterate over the rules do not have to deal with these counters
anymore.
CLEANUP: http: merge error handling for req* and http-request *

The reqdeny/reqtarpit and http-request deny/tarpit were using
a copy-paste of the error handling code because originally the
req* actions used to maintain their own stats. This is not the
case anymore so we can use the same error blocks for both.

The http-request rulesets still has precedence over req* so no
functionality was changed.