[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".

platform/default/sqlite3.cpp

@@ -91,6 +91,11 @@ int64_t Database::lastInsertRowid() const {
     return sqlite3_last_insert_rowid(db);
 }
 
+uint64_t Database::changes() const {
+    assert(db);
+    return sqlite3_changes(db);
+}
+
 Statement::Statement(sqlite3 *db, const char *sql) {
     const int err = sqlite3_prepare_v2(db, sql, -1, &stmt, nullptr);
     if (err != SQLITE_OK) {