Globally replaced all use of time with internal 'clock' package
which provides indirection to time.Now()
Added support for faking clock in Kopia via KOPIA_FAKE_CLOCK_ENDPOINT
logfile: squelch annoying log message
testenv: added faketimeserver which serves time over HTTP
testing: added endurance test which tests kopia over long time scale
This creates kopia repository and simulates usage of Kopia over multiple
months (using accelerated fake time) to trigger effects that are only
visible after long time passage (maintenance, compactions, expirations).
The test is not used part of any test suite yet but will run in
post-submit mode only, preferably 24/7.
testing: refactored internal/clock to only support injection when
'testing' build tag is present
