Use streaming I/O in ANALYZE.

Use streaming I/O in ANALYZE.

The ANALYZE command prefetches and reads sample blocks chosen by a
BlockSampler algorithm. Instead of calling [Prefetch|Read]Buffer() for
each block, ANALYZE now uses the streaming API introduced in b5a9b18cd0.

Author: Nazir Bilal Yavuz 
Reviewed-by: Melanie Plageman 
Reviewed-by: Andres Freund 
Reviewed-by: Jakub Wartak 
Reviewed-by: Heikki Linnakangas 
Reviewed-by: Thomas Munro 
Discussion: https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com