Bug Fixes

• core: Fix CompositeChannelCredentials to no longer use CallCredentials for OOB channels. OOB channels are available for load balancing policies to use to communicate with an LB server. It is mainly used by gRPC-LB. This resolves the incompatibility of the 1.34.0 release with googleapis.com.
• alts: Limit number of concurrent handshakes to 32. ALTS uses blocking RPCs for handshakes. If the handshake server has a limit to number of concurrent handshakes this can produce a deadlock. Limiting to 32 should workaround the problem for the majority of the cases. A later fix will allow handshake RPCs to be asynchronous
• xds: Relocate (shade) all generated code; a few classes had previously been missed
• xds: Fixed an issue when GRPC_XDS_EXPERIMENTAL_NEW_SERVER_API=true where gRPC would request non-existent resources