Maintenance Releace

Resolved issues:

• #8 Made send/receive/handshake cancellation safe.

Other:

• Replaced timestamp_nanos for DateTime with timestamp_nanos_opt and made the conversion from DateTime<Utc> into i64 return i64::MIN (0Np in q/kdb+) when the DateTime<Utc> is not between 1677-09-21T00:12:43.145224192 and 2262-04-11T23:47:16.854775807.