Use of boost::make_shared in qana

For classes:
* lsst::qserv::qana::AggregatePlugin
* lsst::qserv::qana::PostPlugin
* lsst::qserv::qana::QservRestrictorPlugin
Use of boost::make_shared in qana

For classes:
* lsst::qserv::qana::ScanTablePlugin
* lsst::qserv::qana::TableInfoPool
* lsst::qserv::qana::TablePlugin
* lsst::qserv::qana::WherePlugin
* lsst::qserv::qana::Executive