Ignore MM for set_security RPCs

Prior to this commit, a node would respect MM for clustered set_security
requests while still contributing to get_security requests. This would
lead to a situation where a user would change the security header for a
clustered DB, but the update would only reach a subset of the shards. On
subsequent requests, intermittant permission errors would be returned
due to security headers being inconsistent between shards.

BugzID: 28847