When `Errno::EBUSY` is raised during restore, this could indicate that
the directory being restored into is a mountpoint. In this case we
explain the user how to retry the restore.

Instead of renaming existing storage directories to a timestamped
format, create a timestamped directory in a temporary location and move
the individual files and folders inside storage directories to it. This
helps us with nested storage directories.

# This is the 1st commit message:

Add `copy` backup strategy to combat file changed errors

The backup Rake task used to stream data directly from the live
data directory into the backup. Under many circumstances this worked
OK. However, really active instances would experience a 'file changed
as we read it' error - especially with data like the registry. This
now copies the data first, then compresses it. It will take a bit
more disk space while the backup is in progress, but it's a
necessary thing.

# The commit message #2 will be skipped:

#	Add env var

Since version ruby-2.2.0, method `File::exists?` is deprecated.