Emit a ResourceWarning in concurrent.futures executor destructors

[vstinner]

BPO	30244
Nosy	@vstinner, @zhangyangyu

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-09-19.23:11:41.319>
created_at = <Date 2017-05-03.07:18:07.717>
labels = ['3.7', 'library', 'performance']
title = 'Emit a ResourceWarning in concurrent.futures executor destructors'
updated_at = <Date 2018-09-19.23:11:41.318>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2018-09-19.23:11:41.318>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-09-19.23:11:41.319>
closer = 'vstinner'
components = ['Library (Lib)']
creation = <Date 2017-05-03.07:18:07.717>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 30244
keywords = []
message_count = 1.0
messages = ['292849']
nosy_count = 2.0
nosy_names = ['vstinner', 'xiang.zhang']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue30244'
versions = ['Python 3.7']

[vstinner]

Follow-up of issues:

• issue bpo-30110: test_asyncio reports reference leak, forgot to shutdown concurrent.futures.ProcessPoolExecutor()
• issue bpo-30171: Emit ResourceWarning in multiprocessing Queue destructor

I propose to emit a ResourceWarning warning in concurrent.futures executor destructor if it wasn't cleaned up properly, if shutdown() wasn't called.