Use dynamic cast for shared_from_this holder init

Using a dynamic_cast instead of a static_cast is needed to safely cast
from a base to a derived type.  The previous static_pointer_cast isn't
safe, however, when downcasting (and fails to compile when downcasting
with virtual inheritance).

Switching this to always use a dynamic_pointer_cast shouldn't incur any
additional overhead when a static_pointer_cast is safe (i.e. when
upcasting, or self-casting): compilers don't need RTTI checks in those
cases.