refactor: optimize NodeStore object conversion: (#4353)

When writing objects to the NodeStore, we need to convert them from
the in-memory format to the binary format used by the node store.

The conversion is handled by the `EncodedBlob` class, which is only
instantiated on the stack. Coupled with the fact that most objects
are under 1024 bytes in size, this presents an opportunity to elide
a memory allocation in a critical path.

This commit also simplifies the interface of `EncodedBlob` and
eliminates a subtle corner case that could result in dangling
pointers.

These changes are not expected to cause a significant reduction in
memory usage. The change avoids the use of a `std::shared_ptr` when
unnecessary and tries to use stack-based memory allocation instead
of the heap whenever possible.

This is a net gain both in terms of memory usage (lower
fragmentation) and performance (less work to do at runtime).

src/test/nodestore/Basics_test.cpp

@@ -56,10 +56,9 @@ public:
 
         auto batch = createPredictableBatch(numObjectsToTest, seedValue);
 
-        EncodedBlob encoded;
         for (int i = 0; i < batch.size(); ++i)
         {
-            encoded.prepare(batch[i]);
+            EncodedBlob encoded(batch[i]);
 
             DecodedBlob decoded(
                 encoded.getKey(), encoded.getData(), encoded.getSize());