Re: allow changing autovacuum_max_workers without restarting

I spent sometime reviewing/testing the POC. It is relatively simple with a lot
of obvious value. 

I tested with 16 tables that constantly reach the autovac threashold and the
patch did the right thing. I observed concurrent autovacuum workers matching
the setting as I was adjusting it dynamically.

As you mention above, If there are more autovacs in progress and a new lower setting 
is applied, we should not take any special action on those autovacuums, and eventually 
the max number of autovacuum workers will match the setting.

I also tested by allowing user connections to reach max_connections, and observed the 
expected number of autovacuums spinning up and correctly adjusted.

Having autovacuum tests ( unless I missed something ) like the above is a good 
general improvement, but it should not be tied to this. 

A few comments on  the POC patch:

1/ We should emit a log when autovacuum_workers is set higher than the max.

2/ should the name of the restart limit be "reserved_autovacuum_workers"?

Regards,

Sami Imseih
AWS (Amazon Web Services)