runMigrationSilent is not threadsafe

[NorfairKing]

Example errors:

/dev/null: hDuplicateTo: resource busy (Device or resource busy)

<stderr>: hDuplicateTo: resource busy (Device or resource busy)

This is because of the call to hSilence [stderr] which is not threadsafe.
This call was added in response to #105 because of the way the postgres backend works.
I'm using sqlite, and with that call removed in #965, everything "Just works":tm: but that doesn't solve the problem for postgres.
#105 mentions that you can silence postgres by setting PGOPTIONS to --client-min-messages=warning.

So a few options:

• Leave runMigrationSilent non-threadsafe. Then I'll use runMigration and just get a bunch of nonsense output:
  
• Split runMigrationSilent into a version that uses hSilence and one that doesn't.
• Remove the call to hSilence and instruct postgres users to set PGOPTIONS accordingly.

I'm in favour of the second, with some good docs.
I could do that as part of #965

[parsonsmatt]

Fixed by #971

[NorfairKing]

Wonderful!