Partition numbers

[dhardy]

Currently,

• Partition remembers its number (required)
• Repo uses these numbers in its index (sensible)
• PartitionIO does not know about these numbers, even though they may be in the file names
• Creating a partition requires a PartitionIO and a number
• Loading a partition lets the Partition discover the number from the file contents (where it probably should be anyway)
• RepoIO implementations do know about these numbers

This is historical: it was not planned that the numbers would be found in the file names, and single-partition usage was implemented before multi-partition functionality. This could still be changed. But should it?

Proposals:

1. do nothing
2. make the number available from PartitionIO and just verify when loading; in this case Partition::open() doesn't need the number passed any more

[dhardy]

Closing for now: PartitionIO optionally stores the PartId, Partition requires a PartId on creation.

Possibly Partition should allow creation without the PartId and discover later, but having it like this before was a nuisance.