title: Modern Relay - ConnectionConfig for multiple connections
body: <p>EDIT: after some research I found out that ReactRelayPaginationContainer does only support one <code>@connection</code> for now (relay release 1.0.0).</p>
  
  <p>I am not sure how to deal with multiple Connections in a PaginationContainer using Relay Modern as the only examples I saw so far only included one Connection. To illustrate my concern, there is some code below where I am having troubles </p>
  
  <pre><code>export default createPaginationContainer(MyComponent,
  {
      item: graphql`fragment mycomponent_item on Item {
          foos(first: $count_foo, after: $cursor_foo) @connection(key: item_foos) {
              pageInfo {
                  hasNextPage
                  endCursor
              }
              edges {
                  node {
                      id
                      ...foo_foo
                  }
              }
          }
          bars(first: $count_bar, cursor: $cursor_bar) @connection(key: item_bars) {
              pageInfo {
                  hasNextPage
                  endCursor
              }
              edges {
                  node {
                      id
                      ...bar_bar
                  }
              }
          }
          }
      }`
  },
  {
      direction: 'forward',
      getConnectionFromProps(props) {
          return props.item.foos &amp;&amp; props.item.bars;
      },
      getFragmentVariables(prevVars, totalCount) {
          return {
              ...prevVars,
              count: totalCount,
          };
      },
      getVariables(props, {count, cursor}, fragmentVariables) {
          return {
              count,
              cursor,
  
          };
      },
      query: graphql`
          query itemPaginationQuery(
              $count_foo: Int!
              $cursor_foo: String
              $count_bar: Int!
              $cursor_bar: String
          ) {
              Item {
                  ... mycomponent_item
              }
          }
      `
  });
  </code></pre>
  
  <p>The problem is in the ConnectionConfig of <a href="https://github.com/facebook/relay/blob/master/packages/react-relay/modern/ReactRelayPaginationContainer.js" rel="nofollow noreferrer">PaginationContainer</a> of Relay Modern:</p>
  
  <pre><code>export type ConnectionConfig = {
   direction?: 'backward' | 'forward',
   getConnectionFromProps?: (props: Object) =&gt; ?ConnectionData,
   getFragmentVariables?: FragmentVariablesGetter,
   getVariables: (
     props: Object,
     paginationInfo: {count: number, cursor: ?string},
     fragmentVariables: Variables,
   ) =&gt; Variables,
   query: GraphQLTaggedNode,
  };
  </code></pre>
  
  <p>It is clearly made just for one Connection in the PaginationContainer. And createPaginationContainer only accepts one of such ConnectionConfigs in it's constructor.</p>
  
  <p>How am I supposed to deal with multiple Connections in one PaginationContainer?</p>