Remove -checktime, instead honor `expires` in JSON about stayrtr

[job]

made some progress

vurt$ ./cmd/stayrtr/stayrtr -cache=/var/db/rpki-client/json -checktime=true
ERRO[0000] Error updating: VRP JSON file is older than 24 hours: 2020-07-17 13:38:44 +0000 UTC
INFO[0000] StayRTR Server started (sessionID:13855, refresh:3600, retry:600, expire:7200)
^C

vurt$ ./cmd/stayrtr/stayrtr -cache=/var/db/rpki-client/json -checktime=false
INFO[0000] New update (971 uniques, 971 total prefixes). 0 bytes. Updating sha256 hash  -> 2e25e354abf2c393784daf1e6ca4f7cff5b013abc81a9ccff6ed9818244d0b95
INFO[0000] Updated added, new serial 0
INFO[0000] StayRTR Server started (sessionID:21149, refresh:3600, retry:600, expire:7200)

from stayrtr.

[randomthingsandstuff]

What about when expires key doesn't exist? Wasn't this added to up-rev rpki-client code only? Not sure if this will be problematic from xversion perspective.

[job]

We have a few (not mutually exclusive) options:

• If expires does not exist anywhere in the JSON, issue a warning "please use a modern validator"
• Use metadata->buildtime + (24*60*60)as poor man's substitute. That variable has been around for long time