MINOR: lb: make LB initialization even more declarative

MINOR: lb: make LB initialization even more declarative

This lets lb_ops specify the conditions necessary to bind to this set of
ops. The condition is expressed as a list of mask and match fields on
the algorithm flags. This is then used in proxy_finalize() to locate the
lb_ops corresponding to the current configuration, by iterating  over
the list of lb_ops structures. This list is implemented using the same
mechanisms used for configuration keywords: an INITCALL1 macro to a
registration function.

This also moves the lookup and property flags into the lb_ops structure
that were previously applied manually on a case by case basis.