grpclb: refactor main GRPCLB logic out of GrpclbLoadBalancer. (#3369)

GrpclbLoadBalancer can work in non-GRPCLB (delegate) mode according to
name resolution results.  Previously the policy selection, delegation
and GRPCLB logic are in the same file, which is not very readable.  It
will get worse as we going to implement policy fallback logic soon.
This PR refactors the GRPCLB logic out, and makes GrpclbLoadBalancer
focus on the policy selection and delegation logic.