Refactor GetThreads method on VirtualMachine.cs

[thaystg]

Following this fix https://github.com/mono/debugger-libs/pull/300

We should start trying to async-ify more of debugger-libs.

@DavidKarlas : Most correct way to fix this issue was to change return type to Task<IList<ThreadMirror>> which would allow us to create TaskCompletionSource and following calls would get same Task as 1st call... Ofc this means whole callstack will need to be asynced...