All backends are expected to support the exact argument. A friendly log message
will be printed to prompt users to upgrade backends that fail due to this.
Backends that still implement find_exact will be called without exact as an
argument to search, and we will continue to use find_exact. Please remove
find_exact from such backends and switch to the new search API.
Functionality has been replaced with an `exact` param in the search method.
Backends that still implement find_exact will continue being called via
the old method for now.
This is needed as otherwise each and every backend needs to handle the fact
that some "bad" clients might send {'field': 'value'} instead of
{'field': ['value']} Though the real problem isn't the clients but our
organically grown query API.
All uris given to lookup should be in the result even if there is no
backend to handle the uri, and the lookup result thus is an empty list.
As a side effect, future.get() is now called in the order of the URIs
in the `uris` list, making it easier to mock out
backend.library.lookup() in core layer tests.
For now this doesn't add any corresponding APIs to backends, or for that matter
tracklist.add(uris). This is just to get the API in for clients in 0.20.
