message: Integration message support.
diff: import { ICommit, ICommitDetails } from "@prague/gitresources";
  -import { IDocumentAttributes, MessageType } from "@prague/runtime-definitions";
  +import { IDocumentAttributes, IDocumentSystemMessage, MessageType } from "@prague/runtime-definitions";
  import * as moniker from "moniker";
  import * as winston from "winston";
  import * as core from "../core";
  @@ -181,12 +181,10 @@ export class DocumentStorage implements core.IDocumentStorage {
  tenantId,
  };
  - const integrateMessage: core.IRawOperationMessage = {
  - clientId: null,
  - documentId: id,
  - operation: {
  + const operation: IDocumentSystemMessage = {
  clientSequenceNumber: -1,
  contents: null,
  + data: JSON.stringify(contents),
  metadata: {
  content: contents,
  split: false,
  @@ -194,7 +192,12 @@ export class DocumentStorage implements core.IDocumentStorage {
  referenceSequenceNumber: -1,
  traces: [],
  type: MessageType.Fork,
  - },
  + };
  +
  + const integrateMessage: core.IRawOperationMessage = {
  + clientId: null,
  + documentId: id,
  + operation,
  tenantId,
  timestamp: Date.now(),
  type: core.RawOperationType,
  