fix: adds multithread protections for LDClient.start(...)

[tanderson-ld]

Requirements

• [ ] I have added test coverage for new or changed functionality Adding unit tests for concurrency issues isn't definitive. Issues such as this need to be thought about at design time.

• [x] I have followed the repository's pull request submission guidelines

• [x] I have validated my changes against all supported platform versions

Related issues

https://github.com/launchdarkly/ios-client-sdk/issues/381

Describe the solution you've provided

Simplified instance dictionary modifications during start and added queue/lock protection for instances.