Add RPCLoopFormContract and RPCLoopRenewContract

This implements contract formation and renewal functionality in the new RPC loop. Unlike the other RPCs, there are two requests/responses. The renter will automatically use the new protocol in FormContract/RenewContract if the host's reported version is v1.4.0 or above. (This has implications for versioning the renter-host protocol -- currently, we are using the host's reported siad version to identify which version of the renter-host protocol it supports.)