Global Validation

Links and validation constraints might point to keys not loaded with the respective kdbGet.

• no workaround such as kdb get -a

• global plugin can register additional backends to load
• split kdbGet into multiple steps:
  1. do kdbGet on system/elektra to update mount points
  2. first do kdbGet on the spec/-namespace
  3. then calculate which backends are needed
  4. then fetch all backends as needed
• split loops in kdbGet not only according placements but also according namespace (get spec first)

• always load everything (with mmap)

• Internal Cache

see #1291