Allow SLOs to be made up of other SLOs

We've gone back and forth about this. In the initial commit, it couldn't
possibly work because you wouldn't be able to get the Etags to match. Then it
was expressly disallowed with a custom error message, and now its allowed. The
reason we're allowing it is that 1,000 segments isn't enough for some use cases
and we decided its better than just upping the number of allowed segments. The
code to make it work isn't all that complicated and it allows for virtually
unlimited SLO object size. There is also a new configurable limit on the
maximum connection time for both SLOs and DLOs defaulting to 1 day. This will
hopefully alleviate worries about infinite requests. Think I'll leave the
python-swift client support for nested SLOs to somebody else though :).

DocImpact

Change-Id: Id16187481b37e716d2bd09bdbab8cc87537e3ddd
Merge "Allow SLOs to be made up of other SLOs"