reconnect redis on forking for switchman

fixes CNVS-15308

test plan:
 * have multiple shards
 * in console:
   Rails.cache.write('a', 'b')
   Shard.with_each_shard(parallel: 2) { Shard.default.activate { puts Rails.cache.fetch('a') } }
 * you should see a bunch of b's (one per shard), and no errors

Change-Id: I5ad7b3ae7d6c03005d229f04cf5bc2ee1e51b948
Reviewed-on: https://gerrit.instructure.com/40832
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>