Loading DefaultSampler slows down cold start when using in Lambda

[maxday]

👋 Hello team!

Both LocalSampler and DefaultSampler are mutually exclusive and cannot be used at the same in core/recorder.py
The fact that they are both imported at the very beginning of the file slows down cold start when using this SDK in a Lambda function.

DefaultSampler is quite slow to load and counts for about 6% of the cold start duration in our use case (see the detailed flame graph attached, purple section)

I will propose a PR to delay import in if/else so DefaultSampler won't be loading while using in Lambda.

[maxday]

Closing this one as #312 has been merged :)