BaseProxy cache should be cleaned when Manager client is reconnected

[09555990-614a-4f9d-a423-b7421fb96627]

BPO	34766
Nosy	@pitrou, @applio, @wynfred
PRs	python/cpython#9476

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['3.7', 'type-bug', 'library'] title = 'BaseProxy cache should be cleaned when Manager client is reconnected' updated_at = user = 'https://github.com/wynfred' ``` bugs.python.org fields: ```python activity = actor = 'wynfred' assignee = 'none' closed = False closed_date = None closer = None components = ['Library (Lib)'] creation = creator = 'wynfred' dependencies = [] files = [] hgrepos = [] issue_num = 34766 keywords = ['patch'] message_count = 5.0 messages = ['326388', '329480', '331765', '333896', '341631'] nosy_count = 3.0 nosy_names = ['pitrou', 'davin', 'wynfred'] pr_nums = ['9476'] priority = 'normal' resolution = None stage = 'patch review' status = 'open' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue34766' versions = ['Python 2.7', 'Python 3.6', 'Python 3.7'] ```

[09555990-614a-4f9d-a423-b7421fb96627]

BaseProxy class in multiprocessing has an internal cache _address_to_local that is only cleaned when Manager server gets shutdown. If a Manager is used as client, the cache should to be cleaned when a new connection is created by Manager.

[09555990-614a-4f9d-a423-b7421fb96627]

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[09555990-614a-4f9d-a423-b7421fb96627]

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[09555990-614a-4f9d-a423-b7421fb96627]

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[09555990-614a-4f9d-a423-b7421fb96627]

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