fix cross-shard conversations

cross-shard conversations caused a error when configured to access a
master/slave database setup

test plan:
 * with two users, in separate shards
 * send a message from one user to the other
 * the second user should receive the message successfully

fixes CNVS-15084

Change-Id: If4bde0bf966d44cb94fa1fd23f09190b6fc1e88e
Reviewed-on: https://gerrit.instructure.com/41075
Reviewed-by: Mark Severson <markse@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>