Add ability to coexist with pyepics

[coretl]

To ensure some level of interoperability, aioca will only create (and destroy) its own CA context if there is not one set for the asyncio event loop thread. This means that it is safe to:

• Use aioca and pyepics in different threads
• Use aioca and pyepics in the same thread as long as pyepics is imported before
  aioca is first used

[codecov]

Codecov Report

Merging #36 (7f157cd) into master (146dbaf) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #36   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         3    +1     
  Lines          428       446   +18     
=========================================
+ Hits           428       446   +18     

Impacted Files	Coverage Δ
aioca/_catools.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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