* subset lb: allow ring hash/maglev LB to work with subsets

Skip initializing the thread aware LB for a cluster when the subset
load balancer is enabled. Also adds some extra checks for LB policies
that are incompatible with the subset load balancer.

Risk Level: low
Testing: test additional checks
Docs Changes: updated docs w.r.t subset lb compatibility
Release Notes: n/a
Fixes: #7651

Signed-off-by: Stephan Zuercher <zuercher@gmail.com>