Consider making ReadConcernMajorityNotAvailableYet a retryable error

This came up during testing for Safe Replica Set Reconfig.

During a safe reconfig, the primary will drop snapshots after writing down a new config document. If a read is issued on this node before it updates its snapshot, the server fails with ReadConcernMajorityNotAvailableYet.

The node should eventually be able to update the committed snapshot through heartbeats (2 second interval), so the read will eventually succeed. It seems like we should treat this as a retryable error.