Reject timestamps that are unreasonably large.

Set an arbitrary threshold (some time in 2020) beyond which we don't
accept data points.  This is to help catch buggy collectors that
produce timestamps too far out in the future.  It would also have
helped in a recent issue where, in a virtualized environment, the
guest's clock jumped forward 40 years momentarily, and caused some
collectors to produce impossibly high timestamps that were stored
by tcollector, and caused tcollector to discard all subsequent data.