Change write connection to read connection in attachMetadatabase

[davidrothera]

When connections are created inside GRDB DatabasePool instances, they are done lazily for both read and write connections.

At the moment, in attachMetadatabase, we are creating a short-lived DatabasePool and instantly using it for a write connection to execute SELECT 1 in order to ensure the database exists and is functional.

The issue with this is that write connections require an exclusive lock on the file; usually, this is not an issue as when using a single DatabasePool instance, these are queued serially, but if one of these lazy connections is created at the same time as another DatabasePool instance has the database locked, we end up with a SQLite exception:

SQLite error 5: database is locked - while executing `BEGIN IMMEDIATE TRANSACTION`

This PR changes that connection to a read one, which, with WAL enabled databases, can leverage the shared memory file and "play nicely" with other threads.

Fixes #394