Re: refactoring relation extension and BufferAlloc(), faster COPY

I'll continue reviewing this, but here's some feedback on the first two 
patches:

v2-0001-aio-Add-some-error-checking-around-pinning.patch:

I wonder if the extra assertion in LockBufHdr() is worth the overhead. 
It won't add anything without assertions, of course, but still. No 
objections if you think it's worth it.


v2-0002-hio-Release-extension-lock-before-initializing-pa.patch:

Looks as far as it goes. It's a bit silly that we use RBM_ZERO_AND_LOCK, 
which zeroes the page, and then we call PageInit to zero the page again. 
RBM_ZERO_AND_LOCK only zeroes the page if it wasn't in the buffer cache 
previously, but with P_NEW, that is always true.

- Heikki