Change --ponyminthreads default to 0

[dipinhora]

Thanks to @slfritchie's hard work in #2926 and #2561 to find and
fix the bugs in the dynamic scheduler scaling logic, scheduler
thread 0 now also suspends and resumes correctly without issues
(in my testing).

This commit changes the default for --ponyminthreads from 1 to
0. The last time @slfritchie ran into issues was in March and
left a comment with the commands he used at:
#2561 (comment)

I ran the commands from @slfritchie's comment (with minor changes
to ensure --ponyminthreads=0) using Pony 0.26.0 for a few hours
without any issues.

[dipinhora]

The commands I used for testing are the following:

Start data receiver (from wallaroo) to listen on port 9999:
./utils/data_receiver/data_receiver --ponyminthreads=0 --ponythreads=1 --ponynopin --ponynoblock --no-write --framed --listen :9999

Run sender (from wallaroo) to send data every 10 ms to port 9999 for about 15 mins at a time. 10 ms is used to give scheduler thread 0 enough time to sleep between batches.
while true; do date; ./giles/sender/sender -h :9999 -m 45000000 -s 450 -i 10_000_000 -f ~/wallaroo/testing/data/market_spread/nbbo/350-symbols_nbbo-fixish.msg -r --ponyminthreads=0 --ponythreads=1 -y -g 46 --ponynopin -w --ponynoblock; done

Run the message-ubench (from ponyc examples) command that hung for @slfritchie repeatedly:
while true; do date; ./examples/message-ubench/message-ubench --initial-pings=1 --pingers=1 --ponythreads=32 --report-interval=10 --report-count=60 --ponynoblock --ponyminthreads=0 --ponynopin; done

[jemc]

Can we explain what zero means here?

It's not immediately clear to me and the same will likely be true for others.

[dipinhora]

@jemc The 0 means that it allows all scheduler threads to suspend resulting in a more efficient application with no busy polling at all when there is no work to be done (the ASIO thread is still awake and will wake up a scheduler thread when more work needs to be done either due to a timer, or network io, or some other similar event). --ponyminthreads controls the minimum number of scheduler threads that must be active (and busy polling for work), no matter what.

I'm not sure if you meant for me to include what 0 means in the help text. If so, I'm not sure of a succinct way to write that up.

[jemc]

I suggest: Defaults to 0, meaning that all scheduler threads are allowed to be suspended when no work is available

[SeanTAllen]

@dipinhora can you write up the release notes to be included with this?

[dipinhora]

@SeanTAllen Release notes:

Change the --ponyminthreads default so that it allows all scheduler threads (as opposed to all but 1 previously) to suspend resulting in a more efficient application with no busy polling at all when there is no work to be done (the ASIO thread is still awake and will wake up a scheduler thread when more work needs to be done either due to a timer, or network io, or some other similar event).

[SeanTAllen]

This can be merged once the help output is updated.

[dipinhora]

Help output updated and PR rebased onto latest master.

[SeanTAllen]

I'll merge this once it passes CI