Report a correct value for changes_pending

This uses the work done in BugzId: 24236 that provides a new pending
field in _changes feeds. The source _changes is polled with a
`?since=seq&limit=0` query string to get the remaning changes to be
processed for a given sequence.

BugzId: 26015
Rate limit the retrieval of pending changes

This limits the update of the changes_pending field to once every
`connection_timeout` milliseconds.

BugzId: 26015
Remove progress field from replication tasks

The progress field doesn't make much sense in the context of a
replication. The new changes_pending field should be the correct gauge
of how far behind a replication is.

BugzId: 26015
Remove unused unpack_seqs function

This is no longer needed now that we're not calculating progress based
off update_seq values.

BugzId: 26015
Fallback to using source seq for Apache CouchDB

If the update_seq is a number we assume that the source server is Apache
CouchDB so we fall back to comparing the current update_seq against the
update_seq value in the database info blob.

BugzId: 26015
Change term formatting for error logs

HTTP errors were nearly unreadable using the `~p` format after a long
error message. This just uses `~w` so the term is printed on a single
line.
Merge pull request #44 from cloudant/26015-fix-changes-pending-field

26015 fix changes pending field