Don't call sqlite3_config() unless explicitly required to do so

[groue]

Context

GRDB calls the SQLite sqlite3_config function in order to configure the global error log.

Any call to sqlite3_config must happen before any connection to any database. In other words, changing the SQLite configuration after a connection has been opened is a misuse of SQLite .

Bug

GRDB was calling sqlite3_config() once, before opening its first database connection. But issue #283 has revealed that this GRDB practice could conflict with other pieces of code that also open database connections. When GRDB opens its first connection after another part of the program has already opened a connection, this sqlite3_config() invocation is a misuse.

Fix

This PR fixes this, by only calling sqlite3_config() when the GRDB user configures the error log:

// Invokes sqlite3_config
Database.logError = { (resultCode, message) in
    NSLog("%@", "SQLite error \(resultCode): \(message)")
}

This also mean that the user is now responsible for setting the Database.logError before any other piece of code would open a connection.

[groue]

@gereons, I expect this PR to fix #283. Will you eventually give it a try and confirm that the warning does no longer appear?

[gereons]

Will do, sure!

[gereons]

LGTM. Log message is gone, and my app continues to work as intended, I haven't noticed anything unusual.

[groue]

OK @gereons, thanks for your confirmation! I'll see why some test fail on travis, and ship a new version soon after.