Since consecutive calls to rewind() are NOPs, this must invoke next() to trigger a second resume. Additionally, leave a note about how this test might change with PHPLIB-448.

This extracts common logic in next() and rewind() to a new method, which is now also called by resume() after it rewinds the internal iterator.

Remove pre-3.0 compat code in FindAndModify

Since ext-mongodb 1.5.0 drops support for server 2.6, we can remove wire version checks and legacy code for servers before 3.0 (i.e. wire version 3).