adjust constants given new capabilities

The renter is a lot more stable now, and has the ability to replace offline hosts. This means we can safely reduce the redundacny from 6x to 4x, which will help upload times. We have also increased the total number of hosts in use from 24 to 32, which will increase overall reliability relative to the redundancy. Filanny, we have increased the default number of hosts to 42, which is 10 more hosts than are needed to get a chunk to full redundancy. This means that the renter can largely tolerate a lot of failing hosts without the upload being impacted. It also means a greater percentage of hosts on the network will be getting data, which will help with host encouragement.