WIP: Rework access method interface

Hacker,

some time before I proposed patches implementing CREATE ACCESS METHOD.
http://www.postgresql.org/message-id/CAPpHfdsXwZmojm6Dx+TJnpYk27kT4o7Ri6X_4OSWcByu1Rm+VA@mail.gmail.com
As I get from comments to my patches and also from Tom's comment about AM
interface in tablesampling thread – AM interface needs reworking. And
AFAICS AM interface rework is essential to have CREATE ACCESS METHOD
command.
http://www.postgresql.org/message-id/5438.1436740611@sss.pgh.pa.us

This is why I'd like to show a WIP patch implementing AM interface rework.
Patch is quite dirty yet, but I think it illustrated the idea quite clear.
AM now have single parameter – handler function. This handler returns
pointer to AmRoutine which have the same data as pg_am had before. Thus,
API is organized very like FDW.

However, this patch appears to take more work than I expected. It have to
do many changes spread among many files. Also, it appears not so easy to
hide amsupport into AmRoutine, because it's needed for relcache. As a
temporary solution it's duplicated in RelationData.

What do you think about this approach of AM interface rework?

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company