changes with doc ids as post request

This changes the _changes request in the replicator from GET to POST when using doc_ids. The doc_ids array is converted to a JSON object `{ "doc_ids": [<doc_ids>] }`.

It fixes a problem when the list of doc_ids is getting too long to fit into a query parameter and (IMHO) is the correct approach according to the documentation.

Test is included, but it only tests with a mere 500 out of 1000 docs, so its not really exposing the GET-based problem. I did successfully replicate a set of 50000 out of 10000, but this takes a while.