* Add a framed average peer clock skew calculator
* Add config property "router.clockOffsetSanityCheck" to determine
if NTP-suggested clock offsets get sanity checked (default "true")
* Reject NTP-suggested clock offsets if they'd increase peer clock skew
by more than 5 seconds, or make it more than 20 seconds total
* Decrease log level in getMedianPeerClockSkew()