lockfile : wait instead of aborting

Hi,

Feature request - currently btrbk abort if the lockfile is present

Change this (optional or by default) to wait until the lockfile has been removed, then continue

This would be useful for example when connecting multiple backup disks and copying to all of them in parallel
`btrbk archive --override=lockfile=/run/lock/btrbk.lock --override=lockfile_present=wait source target`