[core] Remove use of DispatchWallTime in LambdaContext

[fabianfett]

We should use another type for the deadline in LambdaContext than DispatchWallTime, which we currently use.

[sebsto]

@fabianfett @weissi I'm triaging the issues to address before the release of a v2-beta.
Can you explain why DispatchWallTime is an issue ? I sit because it's not Sendable ?

Can we just use TimeInterval and Date().timeIntervalSince1970 ?

Writing a complete UTCClock implementation (as suggested) for this project seems an overkill.

[weissi]

@fabianfett @weissi I'm triaging the issues to address before the release of a v2-beta.
Can you explain why DispatchWallTime is an issue ? I sit because it's not Sendable

You can't actually get the wall time back from DispatchWallTime. So it's a one-way street which makes it pretty useless.

[sebsto]

Proposed fix in #540