Renter testing + fixes

• the hostDB can communicate with v0.4.8 hosts again. This was broken in 02bddf9b.
• siac was using POST for /renter/download (now GET)
• repair loop now runs in parallel. It spawns a thread for each file. This is probably doesn't scale well. I'm working on capping it to n threads. (done)
• threadedRepairFile checks that there are chunks/contracts in need of repair before opening the file handle.