Specify column cache sizes explicitly; default fallback of 2MB (#2358)

* allow specifying cache sizes for specific columns

* initialize client with correct cache size

* set cache size in client service

* default cache size for unspecified columns

* fix comment

* fix column selection logic

* fix migrations module

util/src/migration/mod.rs

@@ -210,7 +210,7 @@ impl Manager {
 		if migrations.is_empty() { return Err(Error::MigrationImpossible) };
 		let mut db_config = DatabaseConfig {
 			max_open_files: 64,
-			cache_size: None,
+			cache_sizes: Default::default(),
 			compaction: config.compaction_profile,
 			columns: columns,
 			wal: true,