MXS-1475 Add @maxscale.cache.[populate|use]

The earlier @maxscale.cache.enabled has now been replaced with
@maxscale.cache.populate and @maxscale.cache.use that provide
for more flexibility.

With the former it is possible to control in what circumstances
the cache is populated and with the latter one when it is used.
Together they can be used for having a completely client driven
caching.

include/maxscale/session.h

@@ -133,7 +133,9 @@ typedef struct mxs_upstream
  * has been removed. The handler must itself parse the value string.
  *
  * @param context      Context provided when handler was registered.
- * @param name         The variable that is being set.
+ * @param name         The variable that is being set. Note that it
+ *                     will always be in all lower-case irrespective
+ *                     of the case used when registering.
  * @param value_begin  The beginning of the value as specified in the
  *                     "set @maxscale.x.y = VALUE" statement.
  * @param value_end    One past the end of the VALUE.