Added Context::deadline(&self) -> SystemTime

[dcormier]

Description of changes:

Added a function to get the SystemTime value for the deadline of the context.

There have been multiple times that I've needed a SystemTime (or similar) value, rather than simply the epoch ms u64 that Context currently exposes. And I've watched other devs trip over that existing u64 value. This should make it easier to use the deadline.

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[dcormier]

Thanks @calavera!