Resolve "add hwtype and hwaddr_source columns to memfile"

Closes #2236 (closed).

• 8e452b18 add hwtype,hwaddr_source to v6 memfile
• 7afe1511 adapt existing tests to the two columns
• a75e2052 split memfile versions into v4 or v6 because they have diverged some time ago and log messages were being contradictory for v6:

WARN  DHCPSRV_MEMFILE_NEEDS_UPGRADING version of lease file: /tmp/kea-dhcp6.csv schema is earlier than version 4.0
WARN  DHCPSRV_MEMFILE_CONVERTING_LEASE_FILES running LFC now to convert lease files to the current schema: 2.1

• e659243e add tests to check that the versions from the two messages match
• 26f85a67 add a changelog entry
• 717e64d0 add unit tests for complex user contexts - user contexts with two or more entries are not correctly loaded from memfile, see unit test for details; i did not proceed with a fix because it is complex work and deserves its own issue; opened #2257 (closed)