Reorganize the FCV upgrade/downgrade configsvr metadata patch-up around the 2-phase setFCV.

On upgrade:

1. Configsvr finishes invoking setFCV(phase-1) to all shards.
2. Configsvr performs a first metadata patch-up pass to:
   • $set 'timestamp' and 'collectionUUID' to all config.chunks documents
   • create the uuid_* indexes on config.chunks
   • $set timestamp for all config.collections documents
   • Drop ns_* indexes on config.chunks
3. Configsvr invokes setFCV(phase-2) to all shards
4. Configsvr performs a second metadata patch-up pass to:

• • $unset 'ns' from all documents in config.chunks