add if-match support

[John Dickinson]

Add support for If-Match conditional headers on PUTs.

Like If-None-Match, we can only really support * as the value (because of eventual consistency in a distributed system). Attached is a very short start that sketches out where the major pieces need to be added.

[clayg (clay-gerrard)]

needs use case - or we'll probably end up doing it wrong.

[Launchpad Janitor (janitor)]

[Expired for OpenStack Object Storage (swift) because there has been no activity for 60 days.]

[Thomas Herve (therve)]

I had the use case here: https://review.openstack.org/#/c/521468/

I thought it was (wrongly) implemented. We use swift to exchange data, and we need it to do so transactionally. Ended up using the database as a transaction mechanism :/

[Thomas Herve (therve)]

Oh I was using if-match with a value, which may not be the scope of that one.