[shadow] hits open file limits - Refactor

[samyfodil]

ERROR        tau.substrate.service.vm        runtime/shadow.go:51        creating new shadow instance failed with: creating new instance failed with: open /tmp/vm1032816768: too many open files

Can be fixed with sysctl or having the cache for open files or read from memory

[samyfodil]

After looking at the code, a refactor is best.