Cassandra Host Data Source

Cassandra Host Data Source

Added src/share/database/scripts/cql/upgrade_1.0_to_2.0.sh to include host_reservations table in Cassandra.
Updated documentation to indicate that Cassandra now supports host reservations.
Added src/lib/dhcpsrv/cql_host_data_source.cc and cql_host_data_source.h.
Fixed a bug in CfgOption::mergeInternal() where formatted_value_ was not taken into consideration when merging, this is needed for merging the denormalized results on host retrieval in Cassandra. The method is not used elsewhere so there is no regression impact.
Added Cassandra support in HostDataSourceFactory.
Implemented a todo in mysql_host_data_source.cc
Added cql_host_data_source_unittest.cc
Functionality that Cassandra is not capable of like ORDER BY clause is
adjusted in generic_host_data_source_unittest.cc.