Made finding destination shards single threaded
Made finding destination shards single threaded

Signed-off-by: Andres Taylor <c60aac06940d152b513cf6eb3d695fd9afbead9b@squareup.com>

vitessio/vitess,enisoc/vitess,tinyspeck/vitess,mahak/vitess,HubSpot/vitess,tinyspeck/vitess,dcadevil/vitess,mahak/vitess,dcadevil/vitess,HubSpot/vitess,HubSpot/vitess,vitessio/vitess,davygeek/vitess,dcadevil/vitess,HubSpot/vitess,tinyspeck/vitess,mahak/vitess,enisoc/vitess,davygeek/vitess,dcadevil/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,HubSpot/vitess,davygeek/vitess,tinyspeck/vitess,tinyspeck/vitess,enisoc/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,dcadevil/vitess,mahak/vitess,enisoc/vitess,enisoc/vitess,mahak/vitess,vitessio/vitess,davygeek/vitess,dcadevil/vitess,dcadevil/vitess,davygeek/vitess,HubSpot/vitess,davygeek/vitess,HubSpot/vitess,mahak/vitess,enisoc/vitess,mahak/vitess,davygeek/vitess,tinyspeck/vitess,vitessio/vitess,HubSpot/vitess,enisoc/vitess