Strategy htex_auto_scale removing blocks far too slow

[jrueb]

Describe the bug
The "htex_auto_scale" strategy is very slow to remove blocks in case there are more slots than tasks. It removes one block per 5 seconds at most.

To Reproduce
Steps to reproduce the behavior, for e.g:

1. Run the test script
2. Check parsl.log and wait for the first handful of tasks are completed
3. The logs shows that it will need around a minute after the idle time out to remove the blocks, even though all tasks but one have been finished. Most of the blocks are idle.

Expected behavior
All idle blocks are removed as soon as possible. This becomes really important if you are dealing with, say 500 blocks and most of them are idle.

Environment
Parsl 1.2.0, Python 3.8

Test script

import parsl
from parsl.providers import LocalProvider
from parsl.channels import LocalChannel
from parsl.config import Config
from parsl.executors import HighThroughputExecutor
from parsl.app.app import python_app


local_htex = Config(
    executors=[
        HighThroughputExecutor(
            label="htex_Local",
            worker_debug=True,
            max_workers=1,
            provider=LocalProvider(
                channel=LocalChannel(),
                init_blocks=2,
                max_blocks=20,
            ),
        )
    ],
    strategy="htex_auto_scale",
    max_idletime=3,
)
parsl.load(local_htex)


@python_app
def long():
    import time
    time.sleep(10000)


@python_app
def short():
    import time
    time.sleep(1)


a = long()
b = [short() for i in range(20)]
a.result()