next-gen: add reserved keyspaces #9414
Description
Development Task
Keyspace ID is allocated in PD, we better reserve some range of ID and name as reserved one, at least for SYSTEM keyspace in next-gen version:
- Reserve keyspace ID range [0xFFFFFF - 1024, 0xFFFFFF] for internal usage
- Reserve SYSTEM keyspace name
- SYSTEM keyspace is created automatically when nextgen PD start, no need any special config, and its ID is fixed to 1.
Valid keyspace id range is [0, 0xFFFFFF](uint24max, or 16777215)