Returns the PostsQueryManager from the state tree for a given site ID (or
for queries related to all sites at once).

@param  {Object}  state  Global state tree
@param  {?Number} siteId Site ID, or `null` for all-sites queries
@return {Object}         Posts query manager
function getQueryManager( state, siteId ) {
	if ( ! siteId ) {
		return state.posts.allSitesQueries;
	}
	return state.posts.queries[ siteId ] || null;
}