Additional workaround for CONNECTORS-616.  This may slow things down by requiring that each connection be verified alive before it is used, but it does reduce the number of IOExceptions from Solr markedly.

git-svn-id: https://svn.apache.org/repos/asf/manifoldcf/trunk@1437900 13f79535-47bb-0310-9956-ffa450edef68