Improve logic to include originalUniqueKey in sharedUniqueKeys

[htmsousa]

Until now, when building the list of SharedUniqueKeys, the code was selecting only the originalUniqueKeys that existed in ghostUniqueKeys list.

However, as part of the alter table, it is possible to add more unique keys or even composed primary keys with multiple columns.

The new logic keeps the old behaviour (it matches originalUniqueKey with exactly the same ghostUniqueKey) but also supports the cases where the originalUniqueKey is now a subset of one of the new ghostUniqueKeys.

If such a case happens, we can still use the originalUniqueKey as normal because a new ghostUniqueKey with the columns of originalUniqueKey plus some new columns is inherently unique just by the columns in the originalUniqueKey, no matter the value in the new columns of the new unique key.

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Related issue: #1452

Further notes in https://github.com/github/gh-ost/blob/master/.github/CONTRIBUTING.md
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Description

This PR [briefly explain what it does]

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.