…tyle

This results in OnlineFileSource containing precisely the logic we want for reuse by OfflineFileSource, and no more.

…e schema

For AssetFileSource and the node FileSource this was already the case; this makes the other implementations consistent.

…ss the same database

When inserting an cached resource, or removing a region, remove least-recently used resources and tiles, not used by offline regions, until the used database size, as calculated by multiplying the number of in-use pages by the page size, is less than the maximum cache size minus 5 times the page size.

In addition, OfflineDatabase may be configured to ignore cache puts of individual resources larger than a certain size.

This policy is similar but not identical to the former SQLiteCache policy:

* It accounts for offline, by exempting resources required by offline regions from eviction.
* It must delete from two tables (resources and tiles), rather than one. Currently the strategy is naive: evict 50 rows at a time from each table.
* It makes maximumCacheSize and maximumCacheEntrySize completely independent. The SQLiteCache implementation evicted when `usedSize > maximumCacheSize - 2 * maximumCacheEntrySize`. This evicts when `usedSize > maximumCacheSize - 5 * pageSize`.
* It uses a non-unlimited default value for maximumCacheSize: 50 MB. We should have always had a limit in place; "a cache without an eviction policy is a resource leak".

Instead, the eviction policy accounts for the actual size needed for an incoming put.

* Under the hood, SQLite creates surrogate keys (ROWID) anyway. We may as well take advantage of this and use the surrogates for foreign keys as well, since they are simpler and more efficient than compound foreign keys.
* Create indexes for efficient eviction queries

This results in a ~1% increase in database size, which is worth it for reducing complexity and making the tiles and resources tables more similar in structure.