Refactor SimpleSAML_Auth_State::extractPersistentAuthState() to getPersistentAuthData() to avoid confusions around the behaviour of this method.

The state array should not be modified after extracting (getting) the persistent authentication data. This resolves #247.

# By Thijs Kinkhorst
# Via Thijs Kinkhorst
* 'master' of github.com:simplesamlphp/simplesamlphp:
  Add more tests for auth proc filters

Deprecate SimpleSAML_Default::initLogin() and move it to SimpleSAML_Auth_Source as a non-static method.

Move SimpleSAML_Auth_Default::handleUnsolicitedAuth() to sspmod_saml_Auth_Source_SP::handleUnsolicitedAuth() and deprecate the former.

Move SimpleSAML_Auth_Default::extractPersistentAuthState() to SimpleSAML_Auth_State::extractPersistentAuthState() and deprecate the former.

(as suggested by jhaar in issue #120 and used by us)