Proposal- GUC for max dead rows before autovacuum

To mitigate the need for per-table tuning of autovacuum configuration, I'd
like to propose a new GUC for autovacuum_vacuum_threshold_max.

Currently, it seems that I can either set autovacuum_vacuum_scale_factor
much smaller than default on tables with millions of rows, or set a value
globally that means small tables are auto vacuumed rarely.

The default value for this new setting value could be -1 or 0 to disable
the feature, or something like 100,000 perhaps so that tables with more
than 500,0000 tuples are candidates for an autovacuum before they would
with current default values.