This is a first step to checking that the key is valid at the required moment.
The idea here is that, rather than passing VerifyKey objects in and out of the
storage layer, we instead pass FetchKeyResult objects, which simply wrap the
VerifyKey and add a valid_until_ts field.
Storing server keys hammered the database a bit. This replaces the
implementation which stored a single key, with one which can do many updates at
once.
The `@cached` decorator on `KeyStore._get_server_verify_key` was missing
its `num_args` parameter, which meant that it was returning the wrong key for
any server which had more than one recorded key.
By way of a fix, change the default for `num_args` to be *all* arguments. To
implement that, factor out a common base class for `CacheDescriptor` and `CacheListDescriptor`.
