Correct improper handle->getMemory() and item.getMemory() callsites

Summary:
This is the preparation work for `getMemory()`/`getMemoryAs()` APIs (in next diff D30938708):
We want read handle (i.e. `const itemHandle`) always return read-only memory and only write handle (i.e. `itemHandle`) can return mutable memory.

To fix the existing improper use of getMemory(), this diff:
- add `const` when the memory is obtained from read handle
- remove `const` if the memory is obtained from write handle

Reviewed By: stuclar

Differential Revision: D30443160

fbshipit-source-id: 09f8ff9008f949d0d71a42cb461b53f30b941506

Author: jiayuebao

cachelib/datatype/MapView-inl.h

@@ -21,7 +21,7 @@ namespace cachelib {
 template <typename K, typename V, typename C>
 MapView<K, V, C>::MapView(const Item& parent,
                           const folly::Range<ChainedItemIter>& children) {
-  hashtable_ = reinterpret_cast<HashTable*>(parent.getMemory());
+  hashtable_ = reinterpret_cast<const HashTable*>(parent.getMemory());
   numBytes_ += parent.getSize();
   for (auto& item : children) {
     numBytes_ += item.getSize();