blockstore: use alloc for block buffer allocation for now

There is potential for around a 10% to 23% increase to the performance
of block reads by using `allocUnsafe`, however there is already around
a 3 to 6 times increase to the performance, and this optimization can
be added later. While it's safe to use `allocUnsafe` as the number of
bytes read is checked to be the same size as the buffer allocation,
there is a potential for test cases to introduce _other_ behavior
for `fs.read` that may not have the same behavior, though this
isn't currently the case.

lib/blockstore/file.js

@@ -482,7 +482,7 @@ class FileBlockStore extends AbstractBlockStore {
     if (offset + length > blockrecord.length)
       throw new Error('Out-of-bounds read.');
 
-    const data = Buffer.allocUnsafe(length);
+    const data = Buffer.alloc(length);
 
     const fd = await fs.open(filepath, 'r');
     let bytes = 0;