MXS-1624 Provide initial implemenation of QC caching

The mapping from a canonical statement to the query classification
result is maintained by the class QCInfoCache of which there exist
an instance per thread. That way no locking is needed but the
information will be cached multiple times (but that is a smaller
price to pay). Currently the information is stored in a regular
std::unordered_map, which means that the consumed amount of
memory will just keep on growing unless the number of canonical
statements used by clients happens to have an upper bound.

The LRU cache (that provides means for putting a bound on the
amount of memory used and number of items) used in the cache filter
will be generalized and be taken into use here as well.

The key is now the canonical statement itself, which means that
a fair amount of memory will be used. To preserve memory it might
make sense to use a hashed value instead, although that at least
in principle opens up the possibility for unintended collisions.

This feature will also be made configurable.

include/maxscale/query_classifier.h

@@ -426,7 +426,7 @@ typedef struct query_classifier
     QC_STMT_INFO* (*qc_info_dup)(QC_STMT_INFO* info);
 
     /**
-     * Closes a dupped info object.After the info objec has been closed, it must
+     * Closes a dupped info object. After the info object has been closed, it must
      * not be accessed.
      *
      * @param info  The info to be closed.