Fixed bugs

* :jirabug:`PT-1611`: ``pt-archiver`` fails with UTF-8 chars
* :jirabug:`PT-1603`: ``pt-table-sync`` is not considering unsorted enum fields in indexes for calculating chunk boundaries
* :jirabug:`PT-1574`: ``pt-online-schema-change`` fails on UK and NULLs
* :jirabug:`PT-1572`: Better usage of ENUM fields in keys in NibbleIterator
* :jirabug:`PT-1422`: ``pt-mysql-summary`` may get stuck when Time: NULL in processlist

Documentation changes

* :jirabug:`PT-1321`: Add required MySQL privileges to ``pt-online-schema-change`` documentation