OPTIM: vars: remove internal bookkeeping for vars_global_size

OPTIM: vars: remove internal bookkeeping for vars_global_size

Right now we have a per-process max variable size and a per-scope one,
with the proc scope covering all others. As such, the per-process global
one is always exactly equal to the per-proc-scope one. And bookkeeping
on these process-wide variables is extremely expensive (up to 38% CPU
seen in var_accounting_diff() just for them).

Let's kill vars_global_size and only rely on the proc one. Doing this
increased the request rate from 770k to 1.06M in a config having only
12 variables on a 16-thread machine.