MLIBZ-2636: replacing Realm ThreadSafeReference usage

[heyzooi]

Description

Replacing Realm ThreadSafeReference usage since it can't be used inside a transaction

Changes

• Using the primaryKey as a reference instead of a ThreadSafeReference instance

Tests

• Unit test added to cover the use case

[codecov-io]

Codecov Report

Merging #313 into develop will increase coverage by 0.08%.
The diff coverage is 94.64%.

@@             Coverage Diff             @@
##           develop     #313      +/-   ##
===========================================
+ Coverage    85.87%   85.95%   +0.08%     
===========================================
  Files           70       70              
  Lines         8557     8552       -5     
===========================================
+ Hits          7348     7351       +3     
+ Misses        1209     1201       -8

Flag	Coverage Δ
#Mac	83.71% <94.64%> (+0.02%)	⬆️
#iOS	85.88% <94.64%> (+0.08%)	⬆️

Impacted Files	Coverage Δ
Kinvey/Kinvey/Operation.swift	87.83% <ø> (+6.75%)	⬆️
Kinvey/Kinvey/PullOperation.swift	100% <ø> (ø)	⬆️
Kinvey/Kinvey/Persistable.swift	85.34% <100%> (+0.38%)	⬆️
Kinvey/Kinvey/Entity.swift	82.83% <100%> (ø)	⬆️
Kinvey/Kinvey/User.swift	90.25% <100%> (-0.19%)	⬇️
Kinvey/Kinvey/Client.swift	85.32% <100%> (-0.17%)	⬇️
Kinvey/Kinvey/FindOperation.swift	96.68% <100%> (ø)	⬆️
Kinvey/Kinvey/Request.swift	98% <100%> (ø)	⬆️
Kinvey/Kinvey/Kinvey.swift	91.32% <100%> (+0.27%)	⬆️
Kinvey/Kinvey/RealmSync.swift	96% <100%> (ø)	⬆️
... and 7 more

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[vinaygahlawat]

LGTM. Are there any implications for replacing Realm ThreadSafeReference with data migration?

[heyzooi]

@vinaygahlawat no, this is not part of the cache schema