Bug status import should happen outside transaction block

At present bug watches updates are pulled in from remote sources within the transaction block within which those updates will be applied to the database. This is a bad idea since a long-running import process could lead to the transaction staying open for far too long. We have scripts which will kill such transactions and which will, as a result, kill the checkwatches process should such a situation occur.

The imports and updates should happen separately, that is to say that imports should be done before the updates are applied to the database, with database updates happening within a transaction block and the imports happening outside of it.