pgsql: Use streaming read I/O in VACUUM's third phase

Use streaming read I/O in VACUUM's third phase

Make vacuum's third phase (its second pass over the heap), which reaps
dead items collected in the first phase and marks them as reusable, use
the read stream API. This commit adds a new read stream callback,
vacuum_reap_lp_read_stream_next(), that looks ahead in the TidStore and
returns the next block number to read for vacuum.

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Co-authored-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Reviewed-by: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA%2BhUKGKN3oy0bN_3yv8hd78a4%2BM1tJC9z7mD8%2Bf%2ByA%2BGeoFUwQ%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c3e775e608f2a6d0bcfba147bf08a506827cc567

Modified Files
--------------
src/backend/access/heap/vacuumlazy.c | 55 ++++++++++++++++++++++++++++++++----
1 file changed, 49 insertions(+), 6 deletions(-)