EpochTimeExtensions and Clock Internal

[jlegan]

Trying to implement custom cache today I came across two classes that would be nice if they were no longer internal:

Clock (default clock implementation) of IClock EpochTimeExtensions

There may be others but those two required a quick copy, paste and rename for clarity on my end.

[leastprivilege]

I made Clock public. The epoch extensions should be used from a separate library (e.g. IdentityModel).