Fix "destination database is in use " error during database backup

[groue]

EDIT 05/10/2018: The analysis of this PR is wrong. See #427 for the "real" fix.

This PR addresses #424.

The DatabaseWriter.erase() method internally uses the SQLite Online Backup API:

extension DatabaseWriter {
    func erase() throws {
        // In order to erase the content, we backup
        // an empty in-memory database into self:
        DatabaseQueue().backup(to: self)
    }
}

This technique used to fail with an error of code 1 (SQLITE_ERROR), message "destination database is in use" when live SQLite prepared statement would prevent the backup from starting (see the regression tests for the precise failure setup).

It happens that Record methods keep prepared statements in a cache, in order to optimize insertions, updates, deletions, and tests for record existence.

This PR thus clears the statement cache before starting a backup, when we used to do it after a successful backup.

The error "destination database is in use" can still happen during a backup, if the client application keeps its own prepared statements alive. To fix this error, the client application must release all prepared statements before starting the backup.

[groue]

The fix is ready, @bellebethcooper 👍

I want to finish #423 first, though: you will have to keep the workaround we discussed in #424 for a little while.

[bellebethcooper]

No stress, the workaround is just fine. Thanks! 👍