Fix incorrect lock scope in BlockingConnectionPool

[kristjanvalur]

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Description of change

In Issue #2995 a case/traceback was shown where a task is timing out waiting to acquire
the BlockingConnectionPool condition variable. This should never happen, timeouts should happen when waiting
for it (the condition).

The Condition Variable for the pool was being held while a Connection attempt was being made with the resulting
connection object. For many tasks making many initial connects, this effectively serialized the pool and could cause
timeouts to occur even if there is room to create new connections.

This PR:

• makes sure the timeout is only on the wait_for() step of the condition variable.
• only waits for a connection slot, an available connection
• perform the ensure_connection() (actual connect call) outside of the Condition Variable.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (054caf3) 91.37% compared to head (c3f360b) 77.71%.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2997       +/-   ##
===========================================
- Coverage   91.37%   77.71%   -13.66%     
===========================================
  Files         126      126               
  Lines       32608    32617        +9     
===========================================
- Hits        29796    25349     -4447     
- Misses       2812     7268     +4456     

Files	Coverage Δ
redis/asyncio/connection.py	83.16% <80.00%> (-0.76%)	⬇️

... and 25 files with indirect coverage changes

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