configurable checkpoint interval
Include reason for replication failure in _replicator doc

closes COUCHDB-1647
Tests for replicator use_checkpoints option
don't crash if ibrowse is already started

fix #COUCHDB-1986
Support SOCKS5 protocol for replication

Using "socks5" as the protocol in the "proxy" parameter of replication
requests will cause DNS resolution and data transfer to happen via a
SOCKS5 proxy server.

COUCHDB-2025
Handle open_revs retries at a higher level

This patch disables the httpc client retries for the request to stream
document revisions to the replicator.  The retry logic at that level
could end up jumbling together response body data from different
requests and thoroughly confusing the multipart parser.  Moving the
retry logic up a level allows us to start fresh each time.

BugzID: 21367
Bubble up missing_stub exceptions for proper retry

A missing_stub exception is special in that the replicator retries the
request without the atts_since optimmization.

COUCHDB-1901
Negotiate max uri length in response to 414's