OPTIM: vars: do not keep variables usage stats if no limit is set

OPTIM: vars: do not keep variables usage stats if no limit is set

The sole purpose of the variable's usage accounting is to enforce
limits at the session or process level, but very commonly these are not
set, yet the bookkeeping (especially at the process level) is extremely
expensive.

Let's simply disable it when the limits are not set. This further
increases the performance of 12 variables on 16-thread from 1.06M
to 1.24M req/s.