Resolve "Improve FileList no that Files are removed from memory"

FileList was updated to walk the renter's directory and call File on all the siafiles found.

This would allow us to:

• reduce the duplicate code that is in FileList and File
• remove the use of the All() method from the SiaFileSet, All() can now be removed during the Repair Loop MR.
• return an error from FileList
• address the problem of loading all siafiles, returning all siafiles, then closing all siafiles since File would open and close one at a time.

Closes #3312 (closed)