MEDIUM: stick-table: set the track-sc limit at boottime via tune.stick-counters

MEDIUM: stick-table: set the track-sc limit at boottime via tune.stick-counters

The number of stick-counter entries usable by track-sc rules is currently
set at build time. There is no good value for this since the vast majority
of users don't need any, most need only a few and rare users need more.
Adding more counters for everyone increases memory and CPU usages for no
reason.

This patch moves the per-session and per-stream arrays to a pool of a size
defined at boot time. This way it becomes possible to set the number of
entries at boot time via a new global setting "tune.stick-counters" that
sets the limit for the whole process. When not set, the MAX_SESS_STR_CTR
value still applies, or 3 if not set, as before.

It is also possible to lower the value to 0 to save a bit of memory if
not used at all.

Note that a few low-level sample-fetch functions had to be protected due
to the ability to use sample-fetches in the global section to set some
variables.