Clean up database abstractions (#1200)

* Remove redundant method

* Pull out a method out of a struct

* More precise db access abstractions

* Move fake trait method out of it

* cargo fmt

* Fix compilation error after refactoring

* Move another fake method out the Store trait

* Get rid of superfluous method

* Fix refactoring bug

* Rename: SimpleStoreItem -> StoreItem

* Get rid of the confusing DiskStore type alias

* Get rid of SimpleDiskStore type alias

* Correction: A method took both self and a ref to Self

consensus/ssz/src/encode/impls.rs

@@ -256,7 +256,8 @@ macro_rules! impl_for_vec {
                         item.ssz_append(buf);
                     }
                 } else {
-                    let mut encoder = SszEncoder::list(buf, self.len() * BYTES_PER_LENGTH_OFFSET);
+                    let mut encoder =
+                        SszEncoder::container(buf, self.len() * BYTES_PER_LENGTH_OFFSET);
 
                     for item in self {
                         encoder.append(item);