This is the genam.h equivalent of 4c850ece (which removed
heapam.h from a lot of other headers).  There's still a few header
includes of genam.h, but not from central headers anymore.

As a few headers are not indirectly included anymore, execnodes.h and
relscan.h need a few additional includes. Some of the depended on
types were replacable by using the underlying structs, but e.g. for
Snapshot in execnodes.h that'd have gotten more invasive than
reasonable in this commit.

Like the aforementioned commit 4c850ece, this requires adding new
genam.h includes to a number of backend files, which likely is also
required in a few external projects.

Author: Andres Freund
Discussion: https://postgr.es/m/20190114000701.y4ttcb74jpskkcfb@alap3.anarazel.de