Unify previous stateid and stateid

[DanielFonsecaTake]

Changed the way of store previous-stateId and stateId context variables to store its values in a single variable.

StateManager:

• Method GetStateIdAsync now extract the state id from the stateid context variable and treats both the new and old format;
• Method GetPreviousStateIdAsync now tries to extract the previous state id from the stateid context variable, if it is not present, it searches the old previousstateid context varible for a value and returns it;
• Method SetStateIdAsync now receives previousStateId as parameter to set it along with the stateid;
• Method SetPreviousStateIdAsync was deleted as it became useless;

[andreminelli]

Use async/await instead of returning Tasks's directly.
See https://github.com/davidfowl/AspNetCoreDiagnosticScenarios/blob/master/AsyncGuidance.md#prefer-asyncawait-over-directly-returning-task