Use streaming read API in ANALYZE

Hi,

I worked on using the currently proposed streaming read API [1] in ANALYZE.
The patch is attached. 0001 is the not yet merged streaming read API code
changes that can be applied to the master, 0002 is the actual code.

The blocks to analyze are obtained by using the streaming read API now.

- Since streaming read API is already doing prefetch, I removed the #ifdef
USE_PREFETCH code from acquire_sample_rows().

- Changed 'while (BlockSampler_HasMore(&bs))' to 'while (nblocks)' because
the prefetch mechanism in the streaming read API will advance 'bs' before
returning buffers.

- Removed BlockNumber and BufferAccessStrategy from the declaration of
scan_analyze_next_block(), passing pgsr (PgStreamingRead) instead of them.

I counted syscalls of analyzing ~5GB table. It can be seen that the patched
version did ~1300 less read calls.

Patched:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 39.67    0.012128           0     29809           pwrite64
 36.96    0.011299           0     28594           pread64
 23.24    0.007104           0     27611           fadvise64

Master (21a71648d3):

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 38.94    0.016457           0     29816           pwrite64
 36.79    0.015549           0     29850           pread64
 23.91    0.010106           0     29848           fadvise64


Any kind of feedback would be appreciated.

[1]:
https://www.postgresql.org/message-id/CA%2BhUKGJkOiOCa%2Bmag4BF%2BzHo7qo%3Do9CFheB8%3Dg6uT5TUm2gkvA%40mail.gmail.com

-- 
Regards,
Nazir Bilal Yavuz
Microsoft