Protect/Warn Against Validity Periods That Are Too Long

[joepetrowski]

After this PR, BlockHashCount should be exposed in the metadata. This is the maximum validity period in blocks. There should be some check, like:

if BlockHashCount in metadata {
    // ensure validityPeriod < BlockHashCount
} else {
    // log "Warning: maximum validity period unknown for this chain..."
}

[emostov]

@joepetrowski is this still relevant to anything txwrapper{-*}?

[joepetrowski]

Yes, every runtime will have this: https://github.com/paritytech/substrate/blob/e33f3c23cdf4d369bd0d2dbec7b78382ee17a117/bin/node/runtime/src/lib.rs#L149