This is the OSS portion of enterprise PR 3822.
Adds a custom gRPC balancer that replicates the router's server cycling
behavior. Also enables automatic retries for RESOURCE_EXHAUSTED errors,
which we now get for free.
The new balancer is a patched version of gRPC's default pick_first balancer
which removes the behavior of preserving the active subconnection if
a list of new addresses contains the currently active address.