MINOR: counters: add shared counters helpers to get and drop shared pointers

MINOR: counters: add shared counters helpers to get and drop shared pointers

create include/haproxy/counters.h and src/counters.c files to anticipate
for further helpers as some counters specific tasks needs to be carried
out and since counters are shared between multiple object types (ie:
listener, proxy, server..) we need generic helpers.

Add some shared counters helper which are not yet used but will be updated
in upcoming commits.