MEDIUM: counters: support looking up a key in an alternate table

MEDIUM: counters: support looking up a key in an alternate table

sc_* sample fetches now take an optional parameter which allows to look
the key in an alternate table. This is convenient to pass multiple
information for the same key at once (eg: have multiple gpc0 for the
same key, or support being fed complementary information from the CLI).
Example :

    listen front
        bind :8000
        tcp-request content track-sc0 src table local-ip
        http-response set-header src-id %[sc0_get_gpc0]+%[sc0_get_gpc0(global-ip)]
        server dummy 127.0.0.1:8001

    backend local-ip
        stick-table size 1k type ip store gpc0

    backend global-ip
        stick-table size 1k type ip store gpc0