migrations: Increase LockTimeout to wait 5min for migrations

Created by: mrnugget

This fixes #4208 (closed) by increasing the LockTimeout from the default 15s to 5min.

As explained in the comment, when two processes (frontend and management-console) race each other to run the migrations, only the first one gets the database lock. The other processes are blocked, waiting to get the lock.

The "waiting for lock" can happen in two places:

1. when instantiating *migrate.Migrate (because that checks that the version table exists atomically with the same database lock)
2. when running the migrations

The problem in #4208 (closed) was that management-console was blocked at (2), while frontend was running the migrations. Since the customer in question had a larger instance, it was not unlikely that the migration ran for longer than 15s, the default timeout. Once the 15s were over, management-console produced the error reported in the ticket.

This change here increases the timeout to 5min, which seems like a reasonable upper-bound for migration times, even for our larger instances.

What this does not do is prevent multiple processes from racing each other to the migrations, because I think this is actually a good thing. Instead of only one process running the migrations and the others booting up and already starting to read/write to the DB (leading to race-conditions with the migrations), the existing behaviour ensures that all processes that directly access the DB have a consistent view of its schema.