Add Traffic Ops If-Match and If-Unmodified-Since Support in Server and Clients

[rob05c]

Currently, there is a race condition if multiple people PUT an object, they will overwrite each-other without realizing it.

HTTP has a well-established solution for this: If-Match and If-Unmodified-Since.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Match

We should implement this, to prevent races and people accidentally overwriting each other.

It shouldn't be too difficult to implement. We can generate an ETag+Last-Modified header from the resource's last_updated field, and likewise compare them when we get a request with IM or IUS.

It's tempting to add the bug tag, since this is a logical bug, even if it's not a code bug. I won't, but I wouldn't object if someone else did.