add column user_max to history table, as describled in https://www.percona.c…

[hankya]

…om/doc/percona-toolkit/2.0/pt-query-digest.html#cmdoption-pt-query-digest--review-history, this will cause pt-query-digest tool to store user info to this column.

This solves issue #138

[boxcla]

Hi @hankya, thanks for the pull request. Before we can merge it, we need you to sign our Contributor License Agreement. You can do so electronically here: http://opensource.box.com/cla

Once you have signed, just add a comment to this pull request saying, "CLA signed". Thanks!

[hankya]

CLA signed.

[boxcla]

I couldn't find a signed CLA for @hankya. Can you please try again?

[hankya]

Signed again, could you please verify?

[akuzminsky]

@hankya boxcla is a bot. I guess it's waiting for a "CLA signed." comment.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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