RE: [Patch] Optimize dropping of relation buffers using dlist

The patch looks OK.  I think as Thomas-san suggested, we can remove the modification to smgrnblocks() and don't care
wheterthe size is cached or not.  But I think the current patch is good too, so I'd like to leave it up to a committer
todecide which to choose. 

I measured performance in a different angle -- the time DropRelFileNodeBuffers() and DropRelFileNodeAllBuffers() took.
Thatreveals the direct improvement and degradation. 

I used 1,000 tables, each of which is 1 MB.  I used shared_buffers = 128 MB for the case where the traditional full
bufferscan is done, and shared_buffers = 100 GB for the case where the optimization path takes effect. 

The results are almost good as follows:

A. UNPATCHED

128 MB shared_buffers
1. VACUUM = 0.04 seconds
2. TRUNCATE = 0.04 seconds

100 GB shared_buffers
3. VACUUM = 69.4 seconds
4. TRUNCATE = 69.1 seconds


B. PATCHED

128 MB shared_buffers (full scan)
5. VACUUM = 0.04 seconds
6. TRUNCATE = 0.07 seconds

100 GB shared_buffers (optimized path)
7. VACUUM = 0.02 seconds
8. TRUNCATE = 0.08 seconds


So, I'd like to mark this as ready for committer.


Regards
Takayuki Tsunakawa