CDC: Flush batch ordering

CDC can include duplicates (different version) of a key in a flush batch.

KV has always written flush batches to the KVStore sorted as by key, then by seqno. This has been a key part of the flusher de-duplication since its creation.

input order to flusher:

key: b a b a

seq: 1 2 3 4

sorted:

key: a a b b

seq: 2 1 4 3

final set written to KVStore:

key: a b

seq: 2 4

With CDC, the final set can now include duplicates, e.g. if the b keys are from a collection with history enabled the output to the KVStore includes two 'b' keys in descending order

final set written to KVStore:

key: a b b 

seq: 2 4 3

When processing such a batch, magma assumed it was working in ascending key order, e.g. it may ignore b:4 or assume it's older than b:3... (note that the seqnos are mostly abstract for magma).

This MB tracks this issue.

Note that the simplest way forward in the short-term is for KV to sort the final batch by seqno which will get everything functional.

Note note: couchstore already (attempts to) sort the batch we handover, twice! Each id/seq index and couchstore sorts the flush input by id or seq.