Filtered replication doesn't record progress unless change is returned

[garethbowen]

I use filtered replication to reduce the number of records replicated to Pouch. When a change passes the filter function the doc is replicated to Pouch and the local seq is updated which future replications use as the since parameter so already filtered changes can be skipped over.

However when no changes are returned the seq is not updated so subsequent replications use the old value for since. This means each doc can be run through the filter function multiple times until a change is made that passes the filter.

For example if the couchdb seq is 2000 and the local seq is 1000 the replication request will look something like...

.../_changes?style=all_docs&heartbeat=10000&filter=medic%2Fdoc_by_place&id=7E7633CB-C6CD-D85D-9ABD-1C6B7570F000&since=1000&limit=100

Then the server runs all the changes from 1000 to 2000 through the filter and responds with...

{
  "results":[],
  "last_seq":2000
}

pouchdb then ignores the response as there are no results to update.

Modify pouchdb to store the last_seq from the changes response in the replication checkpoint so replication starts from the new seq next time.

[nolanlawson]

fixed by f1989ce