Fix group chat timestamp bugs and tests

bindings/group.go

groupChat/gcMessages.pb.go

groupChat/gcMessages.proto

groupChat/groupStore/dhKeyList.go

groupChat/groupStore/dhKeyList_test.go

groupChat/groupStore/group.go

groupChat/groupStore/group_test.go

groupChat/groupStore/utils_test.go

groupChat/internalFormat_test.go

groupChat/makeGroup.go

groupChat/manager.go

groupChat/manager_test.go

groupChat/publicFormat_test.go

groupChat/receive.go

groupChat/receiveRequest.go

groupChat/receiveRequest_test.go

groupChat/receive_test.go

groupChat/send.go

groupChat/sendRequests.go

groupChat/sendRequests_test.go