Allow limiting maximum document body size

Allow limiting maximum document body size

This is the HTTP layer and some tests. The actual checking is done in couch
application's from_json_obj/1 function.

If a document is too large it will return a 413 response code. The error reason
will be the document ID. The intent is to help users identify the document if
they used _bulk_docs endpoint. It will also help replicator skip over documents
which are too large.

COUCHDB-2992