[master] Lease and host back ends now validate schema version during open

    Merge branch 'trac5629'

diff --cc src/lib/dhcpsrv/cql_host_data_source.cc
Simple merge

diff --cc src/lib/dhcpsrv/cql_host_data_source.h
Simple merge

diff --cc src/lib/dhcpsrv/mysql_host_data_source.cc
Simple merge

diff --cc src/lib/dhcpsrv/mysql_host_data_source.h
Simple merge

diff --cc src/lib/dhcpsrv/pgsql_host_data_source.cc
Simple merge

diff --cc src/lib/dhcpsrv/pgsql_host_data_source.h
Simple merge