compute hash in chunks.

Previously we were loading the entire package file into memory, then
updating the hash. Now we update the hash with chunks of the data as it is read in.
Should fix #259

api/src/package.js

@@ -68,10 +68,17 @@ class Package {
 
         logger.debug('Validating checksums');
         logger.debug(`Assert sha256(pkg.tar.gz) == ${this.checksum}`);
-        const cs = crypto
-            .create_hash('sha256')
-            .update(fss.readFileSync(pkgpath))
-            .digest('hex');
+        const hash = crypto.create_hash('sha256');
+
+        const read_stream = fss.create_read_stream(pkgpath);
+        await new Promise((resolve, reject) => {
+            read_stream.on('data', chunk => hash.update(chunk));
+            read_stream.on('end', () => resolve());
+            read_stream.on('error', error => reject(error))
+        });
+        
+
+        const cs = hash.digest('hex');
 
         if (cs !== this.checksum) {
             throw new Error(`Checksum miss-match want: ${val} got: ${cs}`);