Whitespace fix
Get rid of AM_BACKGROUND_PROCESS global variable

This was achieved by simplifying handling background
processes.
Further simplify files promise verification logic
Remove unused variable
Reimplement PurgeLocalFiles

Previously it was implemented as an evaluation of files promises,
which made files promises' implementation highly entangled.

Existing DeleteDirectoryTree contains all the functionality needed
to implement PurgeLocalFiles without referring to files promises.