Remove metadata canonicalization, and fix some bugs

src/core/census/grpc_filter.c

src/core/channel/compress_filter.c

src/core/channel/http_client_filter.c

src/core/channel/http_server_filter.c

src/core/security/client_auth_filter.c

src/core/surface/call.c

src/core/surface/channel.c

src/core/surface/server.c

src/core/transport/chttp2/stream_encoder.c

src/core/transport/chttp2_transport.c

src/core/transport/metadata.c

src/core/transport/metadata.h

test/core/end2end/tests/metadata.c

test/core/transport/metadata_test.c