OTS — interposed coordinator optimisation

When using interposition it's possible for (and I'd argue it probably
should) a subordinate coordinator to run the two-phase commit protocol on
its registered resources (almost) independently of the root coordinator (or
the coordinator it's registered with). Say the subordinate coordinator gets
a prepare message which it then sends to its locally registered resources,
if one of them returns VoteRollback then the subordinate knows that it will
eventually have to rollback all registered resources. Now it could simply do
nothing more and send VoteRollback up the tree, knowing that it will
eventually get a rollback call to then distributed on to its registered
resources. Alternatively, it could send rollback to all of its resource
before sending the VoteRollback up the tree (and then it could obviously
go).

Although we shouldn't mandate an implementation, it might be useful to
mention the above optimisation in the text. Perhaps during the Implementor's
View.

close issue, see above