Mark a replica as unhealthy when the repl lag is unknown
Mark a replica as unhealthy when the repl lag is unknown

This prevents us from serving queries from the replica that may
be beyond the defined -unhealthy_threshold for the tablet.

Signed-off-by: Matt Lord <5c2d6cb5cc654501c0d86e59cab3211bbda644af@gmail.com>

mahak/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,mahak/vitess