For a transaction that requires two-phase commit due to multiple write shards, omit read-only shards from having to do prepare.

(Also, the following should also be done as part of this ticket, from previous SERVER-41356 which should have been part of this)

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.