multi-page-state-preserving web applications

Changes for 2.3

• Agni::agni_exec would local'ize $PApp::NOW, which would incorrectly disassociate from any imported versions.
• use sessionid to strengthen crypto check for state keys.
• PApp::SCGI::Worker destroys and recreates the EV default loop. That's a bit dirty, but there is no way to save and restore a signal handler in pure perl, so the alternatives seem all worse.
• PApp::SCGI: run PApp handler in a Coro thread instead of in an event handler, to allow handlers to block.
• PApp::SCGI: allow exit code to be provided by overwriting $PApp::SCGI::Worker::WORKER_EXIT.
• PApp::SCGI: slight performance optimisations for papp-scgid.
• PApp::CGI warns did not print a trailing newline.
• add HTTP_AUTHORIZATION header suppoort to PApp::CGI.
• PApp:🤐SQL::Sablotron marked as deprecated.
• PApp:🤐SQL::Sablotron now increases Coro's cctx stack because sablotron uses a lot of stack space.
• setting environment variable PAPP_INSTALL_SKIP_CONFIG=1 will skip config installation and questions during make install.