[465-add-subnet4-update-and-subnet6-update-commands-to-subnet-cmds-hook] Added missing tests and updated doc

[465-add-subnet4-update-and-subnet6-update-commands-to-subnet-cmds-hook] Added cfg_subnet replace -- unit test to do

[465-add-subnet4-update-and-subnet6-update-commands-to-subnet-cmds-hook] Added shared network replace(subnet) -- unit test to do

This prevents segfaults during the server shutdown due to unavailable
hook library heap.

    Merge branch 'master' of gitlab.isc.org:isc-projects/kea

src/lib/dhcpsrv/cb_ctl_dhcp6.cc
    CBControlDHCPv6::databaseConfigApply() - revamped to
    delete objects based on DELETE audit entries

src/lib/dhcpsrv/unittests
    Renamed cb_ctl_dhcp_unittest.cc to cb_ctl_dhcp4_unittest.cc
    cb_ctl_dhcp6_unittest.cc - new file

Added proposed ChangeLog