Split sync document exchange into many POST requests
Current Soledad sync process (inherited from u1db) consists of one GET, one POST and one PUT request, where actual documents exchange between client and server happen during the POST request.
We want to be able to stop and resume syncs, and the first step for that is to split the POST request into one POST request per document sent.
(from redmine: created on 2014-04-16, closed on 2014-06-03)