Hash profile files in larger chunks

The asyncio port added an await between every 2048 byte read, which
roughly doubled the cost of hashing.  imgutil runs entire packed images
through this, and the server pays it on rebase and media import.

Read a megabyte per iteration instead.  That still yields hundreds of
times per gigabyte, so the event loop stays responsive, and sha512 is
independent of the read size, so existing manifests remain valid.

confluent_server/confluent/osimage.py

@@ -1041,11 +1041,11 @@ def copy_file(src, dst):
 async def get_hash(fname):
     currhash = hashlib.sha512()
     with open(fname, 'rb') as currf:
-        currd = currf.read(2048)
+        currd = currf.read(1048576)
         await asyncio.sleep(0)
         while currd:
             currhash.update(currd)
-            currd = currf.read(2048)
+            currd = currf.read(1048576)
             await asyncio.sleep(0)
 
     return currhash.hexdigest()