BUG/MAJOR: sample: correctly reinitialize sample fetch context before calling sample_process()

We used to only clear flags when reusing the static sample before calling
sample_process(), but that's not enough because there's a context in samples
that can be used by some fetch functions such as auth, headers and cookies,
and not reinitializing it risks that a pointer of a different type is used
in the wrong context.

An example configuration which triggers the case consists in mixing hdr()
and http_auth_group() which both make use of contexts :

     http-request add-header foo2 %[hdr(host)],%[http_auth_group(foo)]

The solution is simple, initialize all the sample and not just the flags.
This fix must be backported into 1.5 since it was introduced in 1.5-dev19.
MINOR: stick-table: make stktable_fetch_key() indicate why it failed

stktable_fetch_key() does not indicate whether it returns NULL because
the input sample was not found or because it's unstable. It causes trouble
with track-sc* rules. Just like with sample_fetch_string(), we want it to
be able to give more information to the caller about what it found. Thus,
now we use the pointer to a sample passed by the caller, and fill it with
the information we have about the sample. That way, even if we return NULL,
the caller has the ability to check whether a sample was found and if it is
still changing or not.
BUG/MEDIUM: counters: fix track-sc* to wait on unstable contents

I've been facing multiple configurations which involved track-sc* rules
in tcp-request content without the "if ..." to force it to wait for the
contents, resulting in random behaviour with contents sometimes retrieved
and sometimes not.

Reading the doc doesn't make it clear either that the tracking will be
performed only if data are already there and that waiting on an ACL is
the only way to avoid this.

Since this behaviour is not natural and we now have the ability to fix
it, this patch ensures that if input data are still moving, instead of
silently dropping them, we naturally wait for them to stabilize up to
the inspect-delay. This way it's not needed anymore to implement an
ACL-based condition to force to wait for data, eventhough the behaviour
is not changed for when an ACL is present.

The most obvious usage will be when track-sc is followed by any HTTP
sample expression, there's no need anymore for adding "if HTTP".

It's probably worth backporting this to 1.5 to avoid further configuration
issues. Note that it requires previous patch.