rgw: calculate user manifest

Fixes: #8169
Backport: firefly
We didn't calculate the user manifest's object etag at all. The etag
needs to be the md5 of the contantenation of all the parts' etags.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit ddc2e1a8e39a5c6b9b224c3eebd1c0e762ca5782)

Conflicts:
	src/rgw/rgw_op.cc
rgw: calc md5 and compare if user provided appropriate header

Fixes: #8436
Backport: firefly

This was broken in ddc2e1a8e39a5c6b9b224c3eebd1c0e762ca5782. The fix
resurrects and old check that was dropped.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 9c56c86bdac6bcb8e76c3f04e7d393e4eaadd721)