Add 'callbackSetQueueSize' function

[ggayDiamond]

This function is used to change the callback buffer limit. See Epics AppDevGuide, Callback Queue section (16.2.4)
https://epics.anl.gov/base/R3-14/12-docs/AppDevGuide/node17.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.16%. Comparing base (97cf8d6) to head (9e10f23).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
+ Coverage   88.13%   88.16%   +0.02%     
==========================================
  Files          15       15              
  Lines        1256     1259       +3     
==========================================
+ Hits         1107     1110       +3     
  Misses        149      149              

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