Reorganise reseeding

[torben-hansen]

Call-outs:

To avoid synchronisation issues for codepoints that reads/mutates the ctr-drbg state in #1919, we must reorganise the reseed logic. This decouples the code that reads/mutates the ctr-drbg state and the code that gathers entropy. The latter is not an issue. The former will later be wrapped as critical code needing synchronisation as part of the global zeroisation.

Testing:

Added new tests that verifies we can predict the time of reseeding based on the configured reseed bound.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.56%. Comparing base (ccb97ef) to head (8a0ce84).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           randomness_generation    #1941      +/-   ##
=========================================================
+ Coverage                  78.46%   78.56%   +0.10%     
=========================================================
  Files                        585      585              
  Lines                      97061    98007     +946     
  Branches                   13920    13921       +1     
=========================================================
+ Hits                       76160    77003     +843     
- Misses                     20282    20384     +102     
- Partials                     619      620       +1     

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