This commit fixes a DoS vulnerability for certain APIs using
signature V4 by verifying the content-md5 and/or content-sha56 of
the request body in a streaming mode.
The issue was caused by reading the entire body of the request into
memory to verify the content-md5 or content-sha56 checksum if present.
The vulnerability could be exploited by either replaying a V4 request
(in the 15 min time frame) or sending a V4 presigned request with a
large body.