Add hierarchical interfaces for throttling at different levels

git-svn-id: https://svn.apache.org/repos/asf/manifoldcf/branches/CONNECTORS-829@1550411 13f79535-47bb-0310-9956-ffa450edef68
Thread model for obtaining connections requires that state be kept per thread.

git-svn-id: https://svn.apache.org/repos/asf/manifoldcf/branches/CONNECTORS-829@1550415 13f79535-47bb-0310-9956-ffa450edef68