Separate SqlTransaction monad from SqlQueryT

[brandonchinn178]

The monad that is added to the monad transformer stack and maintains the database connection pool / transaction information should be separate from the monad that executes SQL queries in a transaction.

Effectively, SqlTransaction should be the same thing as persistent's SqlPersistT type, except we want to restrict unsafe IO actions, like forkIO (see #7) or IO actions that aren't rerunnable. This is so that databases where transactions are expected to be retried if serialization fails (e.g. https://www.postgresql.org/docs/9.5/transaction-iso.html).

• Add separate SqlTransaction type
• SqlTransaction should forbid MonadIO instances (using GHC.TypeLits.TypeError)
• Add MonadRerunnableIO class + instance
  • with rerunnableIO and rerunnableM
• Write withTransaction :: SqlTransaction m a -> m a
• Test SqlTransaction does not allow arbitrary IO
• Update docs
• Update README, haddocks, etc.
• Update CHANGELOG