Vacuum: allow usage of more than 1GB of work mem

The attached patch allows setting maintainance_work_mem or
autovacuum_work_mem higher than 1GB (and be effective), by turning the
allocation of the dead_tuples into a huge allocation.

This results in fewer index scans for heavily bloated tables, and
could be a lifesaver in many situations (in particular, the situation
I'm living right now in production, where we don't have enough room
for a vacuum full, and have just deleted 75% of a table to make room
but have to rely on regular lazy vacuum to free the space).

The patch also makes vacuum free the dead_tuples before starting
truncation. It didn't seem necessary to hold onto it beyond that
point, and it might help give the OS more cache, especially if work
mem is configured very high to avoid multiple index scans.

Tested with pgbench scale 4000 after deleting the whole
pgbench_accounts table, seemed to work fine.