Implement DB.Ingest for atomically ingesting a set of externally created sstables. The entries in the sstable are all given the same sequence number using the global-seqnum sstable property. That means the ingested sstables must not self overlap. Sstables should be ingested into the lowest level of the LSM tree possible, though that might mean flushing the memtable and ingesting them into L0.