increase number of dl-pieces used by renter

Increasing from 40 to 60 means that the peak memory usage of the renter for downloads is now potentially 80 MiB higher, but it also means that instead of being bottlenecked by your 4 slowest hosts during download, you are bottlenecked by the 6 slowest hosts during download.

This change is acceptable becaues of memory efficieny gains in the wallet. Future changes to the api, to the downloader scheduling, and to uploader-downloader coordination will allow us to reduce this back down at some point while simultaneously also incresaing download speeds.