Presto: Admit coordinator-only "system queries" when required worker count not satisfied

[findepi]

Given coordinator with

node-scheduler.include-coordinator=false
query-manager.required-workers=1

query

select * from system.runtime.nodes;

will get into WAITING_FOR_RESOURCES state, waiting for a worker.
However, once the worker shows up and query gets admitted, the query executes on coordinator only.
We should not wait for a worker in this case.

Relates to: https://github.com/prestosql/presto/issues/4131

[prakharcode]

Hi, would like to take this up.

[findepi]

@prakharcode thanks!
before starting to code this, let's have agreement if this is really what we want.
cc @dain @sopel39 @martint

[raunaqmorarka]

Past discussion about this: https://github.com/prestosql/presto/pull/2484#discussion_r369927055