Opening a writable database should retry if database is locked

[richard]

Writable databases currently fail to open with a DatabaseLocked? exception if the database is already opened by a different process. In situations where multiple processes (or threads) want to open a database for writing, this forces users to implement a retry mechanism to handle these errors. It might be helpful if databases had an option which would cause them to retry (until a timeout was reached) if the database was locked, to avoid users having to implement retry mechanisms in every application wanting this feature.

Alternatively, it might be better to continue with our current approach of suggesting that users structure applications such that database writes are marshalled into a single location and performed on a single WritableDatabase? object.