Avoid CoWs in grpc-web-to-http2 (#1227)
Avoid CoWs in grpc-web-to-http2 (#1227)

Motivation:

The gRPC Web to HTTP/2 codec attempted to avoid CoWs when mutating its
state.

Modifications:

- Avoid CoWs in the GRPCWebToHTTP2ServerCodec

Result:

Fewer CoWs.
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift