Renter stores files in top level directory

MERGE REQUEST

Changes to Work Flow
A filesDir field was added to the renter that is the new top level directory for the renter files. It is renter/siafiles and defined by the constant SiapathRoot. The folder is created during initPersist for the renter and the upload code has been updated to pre-pend siapaths with r.filesDir instead of r.persist.

Testing Updates
No updates to testing, only fixes to ensure testing still passes.

Modules and Packages Impacted
Renter

This is the first MR that will work on #1122 (closed)