querier_cache: use the reader permit for memory accounting

The querier cache has a memory limit it enforces on cached queriers. For
determining how much memory each querier uses, it currently uses
`flat_mutation_reader::buffer_size()`. However, we now have a much more
complete accounting of the memory each read consumes, in the form of the
reader permit, which also happens to be handy in the queriers. So use it
instead of the not very well maintained `buffer_size()`.

querier.hh

@@ -170,7 +170,7 @@ public:
     }
 
     size_t memory_usage() const {
-        return _reader.buffer_size();
+        return _permit.consumed_resources().memory;
     }
 };