title: pt-online-schema-change failed when creating new tablle, specified key was too long; max key length is 767 bytes issue
body: <p>The version, mysql is InnoDB 5.6.10-log, pt-online-schema-change 3.1.0. table info is</p>
  <pre><code>`email` varchar(225) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  `uid` varchar(255) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  `hotel_brand_id` int(11) DEFAULT NULL,
  
  UNIQUE KEY `index_membership_customers_on_uid_and_hotel_brand_id` (`uid`,`hotel_brand_id`),
  </code></pre>
  <p>My purpose is to add a unique index,</p>
  <pre><code>pt-online-schema-change --recursion-method=none --nocheck-unique-key-change h=$DATABASE_HOST,D=$DATABASE,t=$TABLE,u=$DATABASE_USER,p=$DATABASE_PASSWORD   --alter &quot;ADD UNIQUE INDEX index_membership_customers_on_email_and_hotel_brand_id (email, hotel_brand_id)&quot;   --$RUN --alter-foreign-keys-method=auto
  </code></pre>
  <p>then I met the unique index length issue(<a href="https://stackoverflow.com/questions/1814532/mysql-error-1071-specified-key-was-too-long-max-key-length-is-767-bytes">MySQL Error #1071 - Specified key was too long; max key length is 767 bytes</a>),</p>
  <pre><code>Creating new table...
  Error creating new table: DBD::mysql::db do failed: Specified key was too long; max key length is 767 bytes [for Statement &quot;CREATE TABLE `umami_prod`.`_membership_customers_new` (
  </code></pre>
  <p>It seems this was the cause</p>
  <pre><code>`uid` varchar(255) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  UNIQUE KEY `index_membership_customers_on_uid_and_hotel_brand_id` (`uid`,`hotel_brand_id`)
  </code></pre>
  <p>so I want to resize the columns or change its character set, but whatever I got to do,
  it failed because <code>pt-online-schema-change</code> try to create a new table first, then it failed because of this index length issue in the beginning. I couldn't change anything</p>
  <p>So my question is</p>
  <ol>
  <li>How does the current table support this utfm8 varchar(255), How does it possible?</li>
  </ol>
  <pre><code>UNIQUE KEY `index_membership_customers_on_uid_and_hotel_brand_id` (`uid`,`hotel_brand_id`)
  </code></pre>
  <ol start="2">
  <li>Maybe there are table settings I missed for <code>pt-online-schema-change</code></li>
  <li>Anything I couldn't do, next, it is a 4M rows size table, and since <code>pt-online-schema-change</code> couldn't work because of the above issue, my only option is alter, to lock whole table?</li>
  </ol>
  <p>Thanks for your reply, any hint might help, thanks</p>