Bug fixes

• bazel: fix protobuf sha256 (#3924)
• core: fix regression in v1.9.0 where the user-agent would be like "grpc-java-netty1.9.0" instead of "grpc-java-netty/1.9.0" (#3974)
• netty: workaround Netty regression in v1.9.0 (netty/netty#7639) which caused TLS failures to fail with "UNAVAILABLE: Channel closed while performing protocol negotiation" instead of useful failure information (#4033)
• netty: fix regression in v1.9.0 where using GRPC_PROXY_EXP with the Netty transport would cause an UnresolvedAddressException (#4027). ProxySelector (including the default one that processes -Dhttps.proxyHost) is still known-broken for Netty; this is being tracked in #4029