fix some activation-of-the-wrong-shard issues

 * hoist Relation#exec_queries' activation to to_a so that explain tracking
   uses the correct connection
 * activate for Relation#explain, since exec_queries no longer does it
 * make sure AssociationScope gets the correct connection

Change-Id: I0d75a61443a7f82513f1e8aa8ef0bc6a9ce68e14
Reviewed-on: https://gerrit.instructure.com/40012
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
bump version

Change-Id: I5e819209642be5c87791403e7d666d83a58c93b8