AthenaKernel+AthContiners+StoreGate: Faster DataBucket casting for common cases.

When we cast from a DataBucket, first try the conversion using the static SG_BASES information. This can be unfolded and inlined at compile-time using BaseList::foreach.

Also need to move the CLID defininition for CondContBase from StoreGate to where the class itself is declared; otherwise, we get errors from cling.