Improve transaction instrumentation on ASP.NET Core

[Tyrrrz]

Closes #761
Closes #760

@kanadaj

Things needed:

1. Bind the transaction in the middleware on creation
2. A GetCurrentTransaction() on IHub or ISentryClient
3. Possibly a StartSpan() helper method to avoid fiddling with the transaction
4. Either make CaptureTransaction() internal, or change the parameter type to ITransaction for consistency with public APIs.

1. is done in this PR
2. Scope.GetSpan() was added a few PRs ago which should return the last active (not finished) span (or transaction) on the current scope
3. this is not part of the spec, but I think Scope.GetSpan().StartChild() should be equivalent to that
4. done in previous PR