MINOR: stick-table: implement the sc-add-gpc() action

MINOR: stick-table: implement the sc-add-gpc() action

This action increments the General Purpose Counter at the index <idx> of
the array associated to the sticky counter designated by <sc-id> by the
value of either integer <int> or the integer evaluation of expression
<expr>. Integers and expressions are limited to unsigned 32-bit values.
If an error occurs, this action silently fails and the actions evaluation
continues. <idx> is an integer between 0 and 99 and <sc-id> is an integer
between 0 and 2. It also silently fails if the there is no GPC stored at
this index. The entry in the table is refreshed even if the value is zero.
The 'gpc_rate' is automatically adjusted to reflect the average growth
rate of the gpc value.

The main use of this action is to count scores or total volumes (e.g.
estimated danger per source IP reported by the server or a WAF, total
uploaded bytes, etc).