[release] 1.20.0 - Optimizations & persistent storage updates (#172)

Made optimizations to evaluation logic:
- Unmarshal spec defaultValue and rule returnValue on initialization
(rather than per evaluation)
- Remove unnecessary string conversion for layer assignment
- Replace array contains with map lookup
- Replace `Sprintf` with `strconv`

#### Impact
- Reduces layer evaluation for layers with partial allocation by about
50%
- Reduces general evaluation for dynamic configs, experiments, layers by
about 15%
---
- Refactored persistent storage interface to include experiment name to
solve for concurrent user evaluations across SDK instances


>Included In This Release
>- b171689 kenny-statsig
>   - support persisted layers (#171)
>- 6f1aea8 kenny-statsig
>   - refactor persistent storage (#170)
>- 8fcd077 kenny-statsig
>   - optimize string conversion  (#169)
>- 95a9fd4 kenny-statsig
>   - optimize array contains to lookup (#167)
>- 2eb486f kenny-statsig
>   - optimize parsing config values and layer bucketing (#166)

statsig_metadata.go

@@ -14,7 +14,7 @@ type statsigMetadata struct {
 func getStatsigMetadata() statsigMetadata {
 	return statsigMetadata{
 		SDKType:         "go-sdk",
-		SDKVersion:      "1.19.0",
+		SDKVersion:      "1.20.0",
 		LanguageVersion: runtime.Version()[2:],
 		SessionID:       SessionID(),
 	}