pgsql: Simplify autovacuum work-item implementation

Simplify autovacuum work-item implementation

The initial implementation of autovacuum work-items used a dynamic
shared memory area (DSA). However, it's argued that dynamic shared
memory is not portable enough, so we cannot rely on it being supported
everywhere; at the same time, autovacuum work-items are now a critical
part of the server, so it's not acceptable that they don't work in the
cases where dynamic shared memory is disabled. Therefore, let's fall
back to a simpler implementation of work-items that just uses
autovacuum's main shared memory segment for storage.

Discussion: https://postgr.es/m/CA+TgmobQVbz4K_+RSmiM9HeRKpy3vS5xnbkL95gSEnWijzprKQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/31ae1638ce35c23979f9bcbb92c6bb51744dbccb

Modified Files
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src/backend/postmaster/autovacuum.c | 318 +++++++-----------------------------
1 file changed, 63 insertions(+), 255 deletions(-)