Replace ints and floats with datetime.timedeltas

[MarkZH]

This PR replaces all int and float data that represent time with proper types.

• Duration: bare int and float replaced by datetime.timedelta
• Stopwatch: two calls to time.perf_counter() replaced with Timer and calls to time_since_reset()
• Timer internals: replace time.time() (which can be altered by system clock changes like daylight saving time) with the monotonic time.perf_counter()
• Variables tagged with _ms, _sec, etc.: explicit conversions to timedelta with functions named msec(), seconds(), etc.

The main benefit of these changes is that there is no need to keep track of what units a given int or float variable is using to store its time. There is no more need for * 1000 or / 1e9 or * 60 operations to get numbers into common units. Everything is handled by the timedelta type. The result of seconds(5) + msec(50) is correctly computed without manually coded conversions.

Other bugs fixed:

• test_bot/lichess.py line 84: wtime used instead of btime

Note:
The only time variables that were not converted to datetime.timedelta are the time control variables in the functions that create a new challenge (create_challenge() and choose_opponent()). Since these functions select values from lists in the user's config file and then immediately send them to Lichess.challenge(), the round trip from int to timedelta and back to int seemed pointless.

[AttackingOrDefending]

Instaed of using timedelta / msec(1) which isn't that easy to understand what it does, we can use:

class Timedelta(datetime.timedelta):
    def total_milliseconds(self) -> float:
        return self / msec(1)

Also, to allow us to use years we can use:

def get_timedelta(years=0, days=0, seconds=0, microseconds=0, milliseconds=0, minutes=0, hours=0, weeks=0):
    return Timedelta(days=years * 365 + days, seconds=seconds, microseconds=microseconds, milliseconds=milliseconds,
                     minutes=minutes, hours=hours, weeks=weeks)

and instead of using import datetime we can use from timer import get_timedelta.

[MarkZH]

I opted for a couple more functions instead of a whole new class. It seemed simpler.

Also, the first post of this PR has been updated to explain why the challenge time control variables in the Matchmaking class were not converted to use datetime.timedelta.