String checkOutConnection(String host, Port port, DataNodeId dataNodeId) {
    return getHostPortPoolManager(host, port, dataNodeId).checkOutConnection();
  }
Attempts to check out an existing connection to the (host, port) provided, or returns null if none available.
@param host The host to connect to.
@param port The port on the host to connect to.
@param dataNodeId The {@link DataNodeId} to connect to.
@return connectionId, if there is one available to use, null otherwise.
ambry-network/src/main/java/com/github/ambry/network/ConnectionTracker.java