fix(debugging): Improve user experience when app crashes and dispatch messages to GCD

fix(debugging): Improve user experience when app crashes in debugger
- Log exceptions the same way as outside of debugger
- Additionally warn that app is being kept alive on purpose
- Drop locks before entering loop so that expression evaluation continues to work
- Stop loop when debugger detaches
fix(debugging): Dispatch messages to global GCD queue Incoming debugger messages could cause a breakpoint to be triggered (e.g. invalid evaluation expression with break on exceptions turned on). We deadlocked when we didn't dispatch them and didn't release the current thread, because no other debugger message could be processed.

PR Checklist

[x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
[x] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
[x] You have signed the CLA.
[x] All existing tests are passing: https://github.com/NativeScript/ios-runtime/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
[ ] Tests for the changes are included.

refs #1200, #1201