Fix limit=N for non-admins by counting post-filter

Fix limit=N for non-admins by counting post-filter

Prior to this commit, the _db_updates endpoint would often return less
than N results for limit=N queries, even when there were N changes to
return. This was because the limiting was done by passing the limit
parameter to fabric:changes, which doesn't account for the per-user
filtering done in global_changes_httpd.

This commit adds explicit counting of the filtered changes in
global_changes_httpd and manually ends the request when N post-filter
changes have been seen.

BugzID: 25272