Made the getZoneCount() method from inmemory client a general datasource client
call (which throws NotImplemented by default), as this does seem useful in other
 cases as well.

Now the work starts to pay off; removed the temporary pointer variable (getInMemoryClientP() still returns a pointer but now directly retrieved from the container), and any direct reference to isc::datasrc::InMemoryClient is removed from au
th_srv (but not from tests as of yet)