Fixes to local store location and migration.

[FIXED]     Duplicate dot when adding extension sqlite to local store name.
[FIXED]     Don't try to migrate when the cloud store is not tentative (another device created a cloud store already).
[ADDED]     API for applications to test whether it's safe to seed the cloud store (if they wish to do so manually).
[FIXED]     Create the directory for the local store before opening it in case it doesn't exist yet.