XC bootstrap

Pavan wrote his proposal on XC bootstrap.   Here's some of my idea/comment

1. It's nice to run initdb as independently as possible and register each node after initdb.   It will be even nicer if initdb runs vacuum freeze so that any node can begin with any GXID.   It will make adding nodes safer.
2. He is right that XC configuration makes sense with at least one coordinator registered.   The issues are:
1. Should a coordinator registered at first?  The order of registration can be more flexible.   Because clients target to a coordinator, there will be no problem to have only datanodes initialized and registered at initial phase of the bootstrap.
2. Should datanode be registered to GTM?   What the registration works for?   
3. Coordinator was made a separate node because we thought coordinator and datanode should be different binaries.  Now that they share the same binary what happens if we a node is both coordinator and datanode?   If they're the same, configuration may look simpler.