[core] Implement an eviction policy for OfflineDatabase

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".

1 files changed, 1 insertions, 0 deletions

diff --git a/platform/default/sqlite3.hpp b/platform/default/sqlite3.hpp
index 13e92bf07f..abe83a2d44 100644
--- a/platform/default/sqlite3.hpp
+++ b/platform/default/sqlite3.hpp
@@ -47,6 +47,7 @@ public:
     Statement prepare(const char *query);
 
     int64_t lastInsertRowid() const;
+    uint64_t changes() const;
 
 private:
     sqlite3 *db = nullptr;