Issue #2091511 by Cameron Tod, mcdruid, mpdonadio, David_Rothstein, lokapujya,...

Issue #2091511 by Cameron Tod, mcdruid, mpdonadio, David_Rothstein, lokapujya, stefan.r, Berdir, alexpott, damiankloip, cosmicdreams, das-peter, heddn, xjm, catch, tstoeckler, anavarre, naveenvalecha, tim.plunkett, dawehner: Make cache_form expiration configurable, to mitigate runaway cache_form tables