host.maxContractLen was a misleading constant, it was not the file
contract that was being checked for a size but the transaction set that
contains all of the unconfirmed dependencies of the file contract.

Because the constant was relevant to both the renter and the host, it
made sense to move the constant to the modules package.

The constant is currently based off of the TransactionSetSizeLimit. As
scalability improves on the network, it might make sense to reduce this
number substantially. While there is a weak connection between the two,
there is no inherent reason that the host should be limiting incoming
transactions based on the TransactionSetSizeLimit (other than having it
as an upper bound), and therefore a separate constant is used. An extra
buffer of 1kb is allowed because the transaction set is going to be
expanded during negotiation when details are added and the signatures
are added.

A sanity check has also been added, because it will never make sense for
the MaxFileContractSetLen to be larger than the transaction pool limit.