MINOR: applet: Add a function to finalize frontend appctx startup

MINOR: applet: Add a function to finalize frontend appctx startup

appctx_finalize_startup() may be used to finalize the frontend appctx
startup. It is responsible to create the appctx's session and the frontend
conn-stream. On error, it is the caller responsibility to release the
appctx. However, the session is released if it was created. On success, if
an error is encountered in the caller function, the stream must be released
instead of the appctx.

This function should ease the init stage when new appctx is created.