Introduce a notion of slack time to the update worfklow's fixed notion of time #137

It could be useful to allow for some slack in metadata expiration time during the client update workflow; for example to help ameliorate the impact of issues such as NTP drift, or to help on clients without real-time clocks.

Such a notion of slack time was introduced in #118, but later removed in #136 (as it was specified in a way which caused too much confusion).

Pseudocode for such a feature might look like (h/t @trishankatdatadog):

# T is the slack time value, i.e. 10mins
fixed_time = now(UTC) - T
# language in the spec
not_expired = expiration > fixed_time
# complement
expired = expiration <= fixed_time

If such a feature is desirable, it may be simpler to specify reliably after #121 is completed.