Router should choose first *write* participant shard as transaction recovery shard

Until now, the first participant has been both the recovery shard and coordinator shard.

With the single-write-shard implementation of first sending commit to read-only shards, then sending commit to the write-shard, it is unsafe for the first participant to be used as a recovery shard. This is because the first participant may be read-only.

Instead, the first write participant will be the recovery shard.