Reduce unnecessary stats in minion cache

Refs #14645

Instead of doing two seperate FS operations to check for the existence
of a file and then open in, simply attempt the file open and continue if
it fails. Should save one stat per operation.

This is a relatively minor optimmization but in cases of a master with
many minions, could have noticeable performance improvements when
iterating over tens of thousands of files.

There's still a *lot* of work to be done here but this is a start.

cc: @felskrone