defrag: allow defrag to start during AOF loading

[JimB123]

Addresses #1393

Changes:

• During AOF loading or long running script, this allows defrag to be initiated.
• The AOF defrag test was corrected to eliminate the wait period and rely on non-timer invocations.
• Logic for "overage" time in defrag was changed. It previously accumulated underage leading to large latencies in extreme tests having very high CPU percentage. After several simple stages were completed during infrequent blocked processing, a large cycle time would be experienced.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.89%. Comparing base (7e56488) to head (4688575).
Report is 6 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1420      +/-   ##
============================================
+ Coverage     70.86%   70.89%   +0.03%     
============================================
  Files           118      118              
  Lines         63612    63614       +2     
============================================
+ Hits          45079    45100      +21     
+ Misses        18533    18514      -19     

Files with missing lines	Coverage Δ
src/defrag.c	91.25% <100.00%> (+1.66%)	⬆️

... and 15 files with indirect coverage changes

[ranshid]

LGTM and we also discussed it offline. looks good.