Concurrency support

[DimitarPetrov]

Issue

Currently if a function is called in a goroutine (potentially multiple times) the order of FunctionInvocation and FunctionReturning events can come out of order.

Potential solution

Have a look at https://golang.org/pkg/runtime/#Callers and use CallerFrames to get which function called the instrumented function and not assume linearity in the events slice.