See: https://github.com/travis-ci/travis-ci/issues/5143

The driver (via libmongoc) already does this.

This reverts commit 451f72c4.

This should help avoid failed IPv6 connections when "localhost" resolves to an IPv6 literal and mongod does not have `net.ipv6}} enabled to listen for IPv6.

This allows the selectDatabase() and selectCollection() to be tested (PHPLIB-144).

If we've learned anything from the legacy driver, it's that options arrays will prove more flexible than nullable, positional arguments in the long run.

The legacy driver would return a comma-delimited list of host:port combinations; however, that would require __toString() to invoke topology discovery on new Manager instances. Returning the URI is less invasive.