[release] 1.21.0 - Optimize local mode initialization (#175)

Introduced 2 new options to configure the load strategy of the IP to
country lookup & UA parser.

- `IPCountryOptions`
  - Disabled: Fully disable IP to country lookup
  - LazyLoad: Load in background
  - EnsureLoaded: Wait until loaded when needed
- `UAParserOptions`
  - Disabled: Fully disable UA parser
  - LazyLoad: Load in background
  - EnsureLoaded: Wait until loaded when needed
 
These are 2 of the more expensive processes during initialization of the
SDK outside of network requests.
- If your service does not rely on country based checks from an IP
address, disabling IPCountry is recommended.
- If your service does not rely on parsing the UserAgent, disabling
UAParser is recommended.
- `LazyLoad` is generally recommended to avoid waiting to initialize
these on start up. `EnsureLoaded` will ensure that it's loaded when
either of these are eventually needed for evaluation.


Bug fix:
- Disable internal error logging in Local Mode






>Included In This Release
>- 3b13ae3 kenny-statsig
>   - skip network error handling if in local mode (#174)
>- 9cff2f5 kenny-statsig
>   - optimize synchronous initialization processes (#173)

statsig_metadata.go

@@ -14,7 +14,7 @@ type statsigMetadata struct {
 func getStatsigMetadata() statsigMetadata {
 	return statsigMetadata{
 		SDKType:         "go-sdk",
-		SDKVersion:      "1.20.0",
+		SDKVersion:      "1.21.0",
 		LanguageVersion: runtime.Version()[2:],
 		SessionID:       SessionID(),
 	}