Always choose a write shard as the transaction two-phase commit coordinator

Currently, the first participant is chosen as the two-phase commit coordinator. This means that if that participant ends up being read-only, the two-phase commit will not have the benefit of any of the participants being local.

This would only improve the tail latency of two-phase commit.

It is slightly complicated because in order to choose a write shard, the router has to first know whether the participant has done a write, which means the router cannot piggyback "coordinator: true" on the request to the participant.