BUG/MEDIUM: counters: flush content counters after each request

One year ago, commit 5d5b5d8 ("MEDIUM: proto_tcp: add support for tracking
L7 information") brought support for tracking L7 information in tcp-request
content rules. Two years earlier, commit 0a4838c ("[MEDIUM] session-counters:
correctly unbind the counters tracked by the backend") used to flush the
backend counters after processing a request.

While that earliest patch was correct at the time, it became wrong after
the second patch was merged. The code does what it says, but the concept
is flawed. "TCP request content" rules are evaluated for each HTTP request
over a single connection. So if such a rule in the frontend decides to
track any L7 information or to track L4 information when an L7 condition
matches, then it is applied to all requests over the same connection even
if they don't match. This means that a rule such as :

     tcp-request content track-sc0 src if { path /index.html }

will count one request for index.html, and another one for each of the
objects present on this page that are fetched over the same connection
which sent the initial matching request.

Worse, it is possible to make the code do stupid things by using multiple
counters:

     tcp-request content track-sc0 src if { path /foo }
     tcp-request content track-sc1 src if { path /bar }

Just sending two requests first, one with /foo, one with /bar, shows
twice the number of requests for all subsequent requests. Just because
both of them persist after the end of the request.

So the decision to flush backend-tracked counters was not the correct
one. In practice, what is important is to flush countent-based rules
since they are the ones evaluated for each request.

Doing so requires new flags in the session however, to keep track of
which stick-counter was tracked by what ruleset. A later change might
make this easier to maintain over time.

This bug is 1.5-specific, no backport to stable is needed.