Workaround issues loading session files

ftplugins as ordinary plugins are only loaded once. Though restoring sessions seems to arise issues with the 'tagfunc'.
it is recorded in the session as:

set tagfunc=s:cpptagfunc

but using it resolves the actual scriptname. for example:

:function /cpp
    function <snr>80_cpptagfunc

but after restoring a session a mismatch happens. By using :tag or :tsel it complains that <snr>80_cpptagfunc no longer exists and:

:function /cpp
    function <snr>84_cpptagfunc

that is, the ftpluing was reloaded but 'tagfunc' keeps pointing to the old one.
Fixing this is relatively easy. no longer reference a script function but a global one. in this case I define the function in an autoload/cpptags.vim script. By doing so:

set tagfunc=cpptags#cpptagfunc

and no mismatch is possible.