Apply AreSane() checks to the fees from the network.

'Sane' was already defined by this code as:
fee.GetFeePerK() > minRelayFee.GetFeePerK() * 10000
But sanity was only enforced for data loaded from disk.

Note that this is a pretty expansive definition of 'sane': A 10 BTC
fee is still passes the test if its on a 100kb transaction.

This prevents a single insane fee on the network from making us reject
our stored fee data at start. We still may reject valid saved fee
state if minRelayFee is changed between executions.

This also reduces the risk and limits the damage from a cascading
failure where one party pays a bunch of insane fees which cases
others to pay insane fees.