Add rekey_session and improve init_session in OpenXPKI::Client
Add rekey_session and improve init_session in OpenXPKI::Client

rekey_session generates a new session id on an existing session
init_session now accepts a parameter to create a new session instead
of failing with an error if a requested session can not be loaded

stefanomarty/openxpki,stefanomarty/openxpki,aleibl/openxpki,oliwel/openxpki,openxpki/openxpki,stefanomarty/openxpki,aleibl/openxpki,oliwel/openxpki,oliwel/openxpki,stefanomarty/openxpki,aleibl/openxpki,openxpki/openxpki,stefanomarty/openxpki,stefanomarty/openxpki,aleibl/openxpki,aleibl/openxpki,openxpki/openxpki,aleibl/openxpki,oliwel/openxpki,oliwel/openxpki,openxpki/openxpki,oliwel/openxpki