pgsql: Allow parallel vacuum to accumulate buffer usage.

Allow parallel vacuum to accumulate buffer usage.

Commit 40d964ec99 allowed vacuum command to process indexes in parallel but
forgot to accumulate the buffer usage stats of parallel workers.  This
allows leader backend to accumulate buffer usage stats of all the parallel
workers.

Reported-by: Julien Rouhaud
Author: Sawada Masahiko
Reviewed-by: Dilip Kumar, Amit Kapila and Julien Rouhaud
Discussion: https://postgr.es/m/20200328151721.GB12854@nol

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3a5e22138a8d014590834eb808c99a436c246aab

Modified Files
--------------
src/backend/access/heap/vacuumlazy.c | 47 ++++++++++++++++++++++++++++++++++--
1 file changed, 45 insertions(+), 2 deletions(-)