[4277] Addressed bulk of review comments
src/lib/dhcpsrv/tests/pgsql_exchange_unittest.cc - Added PgSqlBasicsTest test fixture class and tests which exercise all of the PostgreSQL data types we currently use with round-trip database writes and reads src/lib/dhcpsrv/pgsql_connection.cc src/lib/dhcpsrv/pgsql_connection.h - Moved PgSqlResult function impls from .h - Added exception safe implementation of getColumnLabel() to PgSqlResult src/lib/dhcpsrv/pgsql_exchange.cc src/lib/dhcpsrv/pgsql_exchange.h - PsqlBindArray::add() variants which accept raw pointers now throw if the pointer is NULL - PgSqlExchange::getColumnLabel() is now a wrapper around PgSqlResult method src/lib/dhcpsrv/pgsql_host_data_source.h src/lib/dhcpsrv/pgsql_host_data_source.cc - Commentary clean up src/lib/dhcpsrv/pgsql_lease_mgr.cc - Commentary clean up