Mark constantly allocated dest receiver as const.

Mark constantly allocated dest receiver as const.

This allows the compiler / linker to mark affected pages as read-only.

Doing so requires casting constness away, as CreateDestReceiver()
returns both constant and non-constant dest receivers. That's fine
though, as any modification of the statically allocated receivers
would already have been a bug (and would now be caught on some
platforms).

Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de