Allow a certain percentage of slaves to lag

[DrMurx]

This PR allows pt-table-checksum or pt-online-schema-change to continue even if a certain percentage of slave servers is lagging behind.

It addresses setups where, for example, single slaves are continuously taken out of the cluster for OPTIMIZE TABLE or other administrative measures.

This is still a bit of WIP but I wanted to make this feature already available to others in this early stage.

[it-percona]

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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[DrMurx]

I do not work for this company anymore and have neither access to their github repositories nor to the email address the commit was done with.

[svetasmirnova]

Rejecting because CLA is not signed. If you find a way to sign CLA feel free to reopen this PR.