SERVER-14763 Refactor sharding/split_large_key.js

Closes #731

Signed-off-by: Benety Goh <benety@mongodb.com>
(cherry picked from commit c13eefdea68c0d493892014d42d8c4bbe687d4c3)
SERVER-14249 Add mongodump tests with --dbpath

Signed-off-by: Benety Goh <benety@mongodb.com>
(cherry picked from commit 2f0c61885405df346808365dfd2bae70769a9f7e)
Merge remote-tracking branch 'upstream/v2.6' into v2.6