[#2909] Addressed review comments

src/bin/admin/tests/mysql_tests.sh.in
    Added test to verify lease6 address and ipv6 reservation address
    column data conversion

src/lib/dhcpsrv/mysql_host_data_source.cc
    Use isc::asiolink::V6ADDRESS_LEN;

src/share/database/scripts/mysql/dhcpdb_create.mysql
src/share/database/scripts/mysql/upgrade_018_to_019.sh.in
    Add data conversion logic for lease6 and ipv6_reservation columns

src/lib/dhcpsrv/mysql_lease_mgr.cc

@@ -2260,7 +2260,6 @@ MySqlLeaseMgr::createContext() const {
         }
     }
 
-
     // Prepare all statements likely to be used.
     ctx->conn_.prepareStatements(tagged_statements.begin(),
                                  tagged_statements.end());