Remove calls to heapsize (#10432)

* update memorydb trait
* use malloc_size_of instead of heapsize_of
* use jemalloc as default allocator for parity client.

ethcore/db/src/lib.rs

@@ -18,6 +18,9 @@
 
 #![warn(missing_docs)]
 
+extern crate parity_util_mem as mem;
+extern crate parity_util_mem as malloc_size_of;
+
 mod db;
 
 pub mod keys;