saml2_Message: Add unique SessionIndex for each SP.
saml2_Message: Add unique SessionIndex for each SP.

Fixes issue 44.
vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp,vivekrajenderan/simplesamlphp,tectronics/simplesamlphp