Introduced ShardKind

Checkpoint -- began to pass ShardKind into OID.next(), which is where it
will be immediately needed.
Replaced default db with system and user

Checkpoint.  We're still using one actual DB, but with two completely
separate entry points.
OIDs now use a dynamic Shard ID.
Fixed a nasty transaction bug

Was trying to create the Space Thing in System Space; had to rearrange
the code.