CDRIVER-751 check discovered servers in same scan

Bug in unreleased Server Discovery And Monitoring implementation.

The spec requires a single-threaded scan to keep going until all nodes
have been checked, *including* nodes discovered during the scan. But
_mongoc_topology_do_blocking_scan quit once all servers known at the
*start* of the scan were checked. Discovered nodes are added to the
topology description, but not checked until the next scan.

With this patch, a block scan keeps going until all servers are checked,
including those discovered during this scan.

TODO: test_topology_scanner_oscillate currently fails, revealing that
in a pathological scenario the driver now errs on the other side, and
scans forever.