src/share/database/scripts/pgsql/upgrade_020_to_021.sh.in
    - new file, corrects dhcp4_server_modifcation_ts index

configure.ac
    added src/share/database/scripts/pgsql/upgrade_020_to_021.sh

src/bin/admin/tests/pgsql_tests.sh.in
    Added pgsql_upgrade_20_to_21_test()

src/lib/pgsql/pgsql_connection.h
    Updated schema version

src/share/database/scripts/pgsql/.gitignore
src/share/database/scripts/pgsql/Makefile.am
    Added upgrade_020_to_021.sh

src/share/database/scripts/pgsql/dhcpdb_create.pgsql
    Added correction of dhcp4_server_modifcation_ts index