Re: Extensible storage manager API - SMGR hook Redux

Hello!

I rebased the patch and addressed all comments.

> Are we sure we need to force extension authors to implement prefetch?
> Also, do we intentionally skip Assert on smgr_registersync and
> smgr_init here? I am not questioning smgr_shutdown here, as I can see
> it is NULL for md implementation.

Added the new asserts, also for the new startreadv.

I don't see a problem with requiring implementing prefetch, the
implementing function can be empty.

> 0002:
> should we merge this with 0001?

Done!

> 0004:
> It's a bit strange to place fsync_checker under contrib, huh? Like,
> you will never use it in production. Maybe src/test/modules is a
> better place?

I moved it to the test folder. In a later version I'll also modify it
to something more suited for actual testing - for now it's just a
simple example use.

> 0005:
> We are missing rationale for this change in the commit message.

I added a more detailed commit message, which hopefully explains why
this change could be useful.