Matchmaker improvements, support for party matchmaking. (#521)

CHANGELOG.md

go.mod

go.sum

main.go

server/config.go

server/match_registry_test.go

server/matchmaker.go

server/pipeline_matchmaker.go

server/session_ws.go

vendor/github.com/heroiclabs/nakama-common/api/api.proto

.../github.com/heroiclabs/nakama-common/rtapi/realtime.pb.go

.../github.com/heroiclabs/nakama-common/rtapi/realtime.proto

...or/github.com/heroiclabs/nakama-common/runtime/runtime.go

vendor/modules.txt