fix: Fix race condition in document cache

[bstasyszyn]

This commit fixes a race condition which exists in the document cache. The scenario is as follows:

• Resolving peer invalidates a document
• Resolve is called before the document is updated in the store by the Observer
• The old document is cached
• Observer stores the new document but the old document is already cached

In order to fix this race condition, instead of deleting the document from the cache, the document is marked stale. So, the next time it is retrieved from the cache it will show as stale and the document will be loaded from the store.

closes #477

Signed-off-by: Bob Stasyszyn Bob.Stasyszyn@securekey.com

[codecov[bot]]

Codecov Report

Merging #478 (91dcd5e) into master (87db0ae) will decrease coverage by 0.05%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
- Coverage   92.50%   92.45%   -0.06%     
==========================================
  Files          81       81              
  Lines        3242     3286      +44     
==========================================
+ Hits         2999     3038      +39     
- Misses        161      165       +4     
- Partials       82       83       +1     

Impacted Files	Coverage Δ
pkg/context/doccache/doccache.go	96.77% <100.00%> (+2.89%)	:arrow_up:
pkg/observer/processor.go	84.89% <0.00%> (-3.60%)	:arrow_down:

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