MINOR: stktable: implement "recv-only" table option

MINOR: stktable: implement "recv-only" table option

When "recv-only" keyword is added on a stick table declaration (in peers
or proxy section), haproxy considers that the table is only used for
data retrieval from a remote location and not used to perform local
updates. As such, it enables the retrieval of local-only values such
as conn_cur that are ignored by default. This can be useful in some
contexts where we want to know about local-values such are conn_cur
from a remote peer.

To do this, add stktable struct flags  which default to NONE and enable
the RECV_ONLY flag on the table then "recv-only" keyword is found in the
table declaration. Then, when in peer_treat_updatemsg(), when handling
table updates, don't ignore data updates for local-only values if the flag
is set.