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