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TestIssueOptionalAttributeUpdateSchemeManager now also sets up a server
with both configurations and sends the request to verify the server
responds correctly. TestDisclosureNonexistingCredTypeUpdateSchemeManager
now also checks cases were an AttributeType is unknown,
but the credential is known.

The server now responds with a "required attribute missing" error in case
a non-optional attribute is missing from the request, after trying
to redownload the configuration to see if it was tagged optional in the
meantime. This change required to extend the IrmaIdentifierSet with an
extra field that tracks missing AttributeTypeIdentifiers. Code that
populates this struct is also updated accordingly for possible future
consumers.

Add comment about sleep

Closes #44

Null attributes are attributes with bigint value 0, meaning that the issuer skipped that attribute. Instead of aborting the session, the irmaclient now allows disclosures of null attributes, when the verifier requests it. This allows the irmaclient to prove that a particular attribute was not issued to it, and allows verifiers to request optional attributes alongside other attributes.

In particular, combined issuance sessions in which 2 or more attributes are
requested from one particular credential type, due to a miscalculation during
verification of the attributes.

Fixes #36

fix: both irmaserver and irmaclient now early reject session requests referring nonexisting identifiers