Fixed bugs

* :jirabug:`PT-1611`: ``pt-archiver`` failed to output UTF-8 characters.
* :jirabug:`PT-1574`: ``pt-online-schema-change`` failed on tables with a nullable unique key and a row with NULL values.
* :jirabug:`PT-1572`: ENUM fields usage in keys was improved, resulting in higher speed for expressions with sorted ENUM items.
* :jirabug:`PT-1422`: ``pt-mysql-summary`` could hang when NULL values appear in the processlist Time column.
Documentation changes

* :jirabug:`PT-1321`: The required MySQL privileges were detailed in `pt-online-schema-change`` documentation

Changelog
---------

* Fixed bug   PT-1611: pt-archiver fails with UTF-8 chars
* Fixed bug   PT-1574: pt-online-schema-change fails on UK and NULLs
* Fixed bug   PT-1572: Better usage of ENUM fields in keys in NibbleIterator
* Fixed bug   PT-1422: pt-mysql-summary may get stuck when Time: NULL in processlist
* Improvement PT-1321: Add required MySQL privileges to pt-online-schema-change documentation
