Grab an advisory lock for migration as per Heroku recommendations

Hi there,

I saw some advice on https://devcenter.heroku.com/articles/release-phase#design-considerations that suggests grabbing an "advisory lock" on the database before running a migration.

Basically, try `SELECT pg_try_advisory_lock(migration);` at the start of the transaction.
- It if returns `t` then nothing currently has the lock, called 'migration', so proceed.
- If not, fail the migration.
The lock will be release when the transaction commits or rolls back.

I think that would be a one-line change in this module. Good idea?

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