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_types/src/fixed_vector.rs

@@ -196,7 +196,8 @@ where
                 item.ssz_append(buf);
             }
         } else {
-            let mut encoder = ssz::SszEncoder::list(buf, self.len() * ssz::BYTES_PER_LENGTH_OFFSET);
+            let mut encoder =
+                ssz::SszEncoder::container(buf, self.len() * ssz::BYTES_PER_LENGTH_OFFSET);
 
             for item in &self.vec {
                 encoder.append(item);