[Refactor] Remove some unnecessary async/await

[dimson-n]

There are many async functions that doesn't really need to be async (e.g. check preconditions and delegate other operations to subsequent call like some helper).

This PR removes some of unnecessary async/await usage in simple cases.

That may cause slightly improve CPU and memory usage and allows to compiler generate more efficient code. Yes, this is micro-optimizations but every little performance issue touches each library user.

[Misha-133]

lgtm

[Waterball12]

You lose some information for debugging. seems to be fine for overload but in the PR is not always the case. Should consider how worth is slightly improve CPU and memory usage.

https://github.com/davidfowl/AspNetCoreDiagnosticScenarios/blob/master/AsyncGuidance.md#prefer-asyncawait-over-directly-returning-task

[dimson-n]

Good remark, but i'm prefer performance in case of frameworks like D.Net is (just imho).
Exception wrapping guarantees may be matter, but almost all tasks awaited inplace here as i can see.