Cache an exception result when retrieving config values

[bbaldino]

ConfigValueSupplier caches the result of the retrieval so we don't re-run the retrieval code on every access, but if the ConfigValueSuppler#doGet chain throws, we bubble the exception up: a ConfigDelegate propagates the exception all the way up and an OptionalConfigDelegate catches it and returns null. This behavior, though, breaks the caching in ConfigValueSupplier as lazy doesn't cache the exception.

This PR adds a ConfigResult class (which is a very simplified version of kotlin's Result, which still has issues) in which we can store the result of retrieval--be it a value or an exception.

[codecov]

Codecov Report

Merging #19 into master will decrease coverage by 0.44%.
The diff coverage is 63.63%.

@@             Coverage Diff              @@
##             master      #19      +/-   ##
============================================
- Coverage     75.72%   75.27%   -0.45%     
- Complexity       42       43       +1     
============================================
  Files            15       16       +1     
  Lines           173      182       +9     
  Branches         13       14       +1     
============================================
+ Hits            131      137       +6     
- Misses           36       39       +3     
  Partials          6        6              

Impacted Files	Coverage Δ	Complexity Δ
...n/kotlin/org/jitsi/metaconfig/util/ConfigResult.kt	55.55% <55.55%> (ø)	0.00 <0.00> (?)
...g/jitsi/metaconfig/supplier/ConfigValueSupplier.kt	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...ain/kotlin/org/jitsi/metaconfig/SupplierBuilder.kt	100.00% <0.00%> (+6.25%)	8.00% <0.00%> (+1.00%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a81dccf...a22fa54. Read the comment docs.

[bbaldino]

I was confused why the coverage went down here (ConfigResult.kt is fully tested), but it looks like jacoco doesn't show inline functions as covered: jacoco/jacoco#654