Fixes #7052 gevent/eventlet 1 seconds latency

[mrmaxi]

Issue #7052
When there are used asynchronous backend as Redis with pubsub, result of short task (0.1s length, for example) available immediate.

When execute AsyncResult.get() under gevent/eventlet environment, result of the same short (0.1s) tasks became available after 1s, it is too slowely.

Current behavior is result of GH-5974 that prevent event loop blocking by very offen checking task results and very short wait_for length.
Current implementation of greenletDrainer cause wait_for to only return every "timeout" # of seconds, rather than returning immediately.

Solutuion:
I add new internal event _drain_complete_event that rising every time after socket operation result_consumer.drain_events (for example _pubsub.get_message for Redis backend) occurs.
Every wait_for are waiting for this particular event, so it won't cause overhead, because socket operation still not block event loop and let other greenlets executing normaly.

[codecov]

Codecov Report

Merging #7089 (366258e) into master (fe37cd8) will increase coverage by 0.01%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master    #7089      +/-   ##
==========================================
+ Coverage   89.35%   89.37%   +0.01%     
==========================================
  Files         138      138              
  Lines       16744    16759      +15     
  Branches     2212     2211       -1     
==========================================
+ Hits        14962    14978      +16     
- Misses       1551     1552       +1     
+ Partials      231      229       -2     

Flag	Coverage Δ
unittests	89.36% <87.50%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
celery/backends/asynchronous.py	69.26% <87.50%> (+2.59%)	⬆️

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[lgtm-com]

This pull request introduces 1 alert and fixes 2 when merging 366258e into fe37cd8 - view on LGTM.com

new alerts:

• 1 for `__init__` method calls overridden method

fixed alerts:

• 2 for Wrong name for an argument in a call

[auvipy]

thank you, will look into this tomorrow