More performance optimizations (#1649)

* Use tree index for DB

* Set uncles_hash, tx_root, receipts_root from verified block

* Use Filth instead of a bool

* Fix empty root check

* Flush block queue properly

* Expunge deref

util/src/migration/mod.rs

@@ -197,7 +197,6 @@ impl Manager {
 		let config = self.config.clone();
 		let migrations = try!(self.migrations_from(version).ok_or(Error::MigrationImpossible));
 		let db_config = DatabaseConfig {
-			prefix_size: None,
 			max_open_files: 64,
 			cache_size: None,
 			compaction: CompactionProfile::default(),