defrag: use monotime in module interface

[JimB123]

The recent PR (#1242) converted Active Defrag to use monotime. In that change, a conversion was performed to continue to use ustime() as part of the module interface. Since this time is only used internally, and never actually exposed to the module, we can convert this to use monotime directly.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 70.86%. Comparing base (9f8b174) to head (2d8eb22).
Report is 1 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/module.c	0.00%	2 Missing ⚠️
src/defrag.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1388      +/-   ##
============================================
+ Coverage     70.68%   70.86%   +0.18%     
============================================
  Files           118      118              
  Lines         63550    63549       -1     
============================================
+ Hits          44919    45036     +117     
+ Misses        18631    18513     -118     

Files with missing lines	Coverage Δ
src/server.h	100.00% <ø> (ø)
src/defrag.c	89.59% <0.00%> (+0.13%)	⬆️
src/module.c	9.64% <0.00%> (ø)

... and 9 files with indirect coverage changes

[hpatro]

The test failure seems unrelated.