TDatabasePDG global static may be used before initialization

Dear ROOTers,

the "fgInstance" is a global static in TDatabasePDG and not yet "hidden" as a function local static so it may be used too early (for now I observed this only when user code uses TDatabasePDG in static initialization, but it may as well affect ROOT).

I attach a patch which removes the "static TDatabasePDG *fgInstance;" member (which was even streamed...?), bumps the class version, and hides it as a function local static.

Please review the patch, maybe you find an even nicer fix. I could not give it full testing yet since I am hit by ROOT-8264 .

Cheers,
Oliver