Use of boost::make_shared in wdb

Perform only trivial replacements
Some replacements couldn't be performed
for next reasons:
* Backend: constructors are private

* ChunkResourceMgr: constructors are private

* Poisoner: lsst::qserv::wdb::
QueryAction::Impl::Poisoner::Poisoner(const
lsst::qserv::wdb::QueryAction::Impl&) does
not exist