MINOR: lb: infrastructure for declarative initialization

MINOR: lb: infrastructure for declarative initialization

This creates an ops structure for the various callbacks into the LB
algorithms, and defines those ops structure for each algorithm. A new
proxy_init callback is added for the initialization functions called
from proxy_finalize(). Since one of them needs to return an int in case
of an error, we change all the others to also return an int so we have a
uniform type.

No functional changes.