Core db schema patch to support longer assembly names in the mapping_session table.

Created by: tgrego

Description

This is a copy of https://github.com/Ensembl/ensembl/pull/458 against master.

Use case

Describe the problem. Please provide an example representing the motivation behind the need for having these changes in place.

Benefits

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Possible Drawbacks

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Testing

Have you added/modified unit tests to test the changes?

If so, do the tests pass/fail?

Have you run the entire test suite and no regression was detected?