UPDATE on migration's unique key column data loss: fixing

[shlomi-noach]

Fixes #519

This work in progress PR is to tackle #519, where an update to the migration's iterating unique key column can result with data loss.

We will either:

• Solve it completely, or
• Recognize the problem in runtime and throw an error, failing the migration, or
• Update the documentation to note such an update is unsupported.

We'll see how goes.

Initial commit: a test that reproduces the data loss.

[shlomi-noach]

At this stage gh-ost will fail a migration when an UPDATE is made to values of the migration key.
This is not to be merged yet. I want to see if gh-ost can further resolve the issue gracefully.

[shlomi-noach]

3898d49 turns an UPDATE which modifies the migration key into DELETE+INSERT (to clarify, only when columns of migration key are actually changed).

Sending to testing.

[shlomi-noach]

Tests are good in production.