chrono deprecations warning cleanup

Cleans up warnings related to specific errors, mostly from_timestamp moving to a from_timestamp_opt function that returns an options.

Note from_timestamp_opt returns None in the following circumstances:

Returns None on the out-of-range number of seconds (more than 262 000 years away from common era) and/or invalid nanosecond (2 seconds or more).

Where timestamps are hardcoded on initialization (such as being hardcoded with 0s), I've just called unwrap as there's no reason for the function to return None
Where timestamps are initialized dynamically in critical code check for None and return appropriate error
where timestamps are less crucial, (tui, mining) just unwrap_or_default

mimblewimble / grin