Ensure that deprecated methods are fully annotated

Found with Google error-prone.
Remove ByteStreams2.asByteSource

This method is dangerous since all ByteSource should provide a new
InputStream on every call to openStream while the method returns the
same InputStream for non-repeatable Payloads.
Ensure that Content-MD5 has 128 bits

Update many tests Content-MD5 headers and ETag responses.