Changed the fieldInfo and primaryKeyFieldName dictionaries to be immutable.

I saw that there weren't any locks used for the `g_fieldInfo` and `g_primaryKeyFieldName` dictionaries, but you were only writing to them in `openDatabaseAtPath:` anyway, so it didn't matter. Just to be safe though, I thought it'd be a good idea to change these to immutable to prevent any future or third party mistakes.