Implement QSqlQuery::setForwardOnly() for PostgreSQL to reduce memory usage

The current implementation of PSQL plugin doesn't support forward only queries.

This is a problem for applications working with large resultsets - the entire resultset is stored in memory, resulting in high memory usage.

For example, the following code consumes about 200MB:

int value;
QSqlQuery query(db);
query.setForwardOnly(true);
query.exec("select generate_series(1, 10000000)");
while (query.next()) {
    value = query.value(0).toInt();
}

Postgresql 9.2 introducted SingleRowMode that can be used to implement setForwardOnly() feature.

Here is code that works the same as above example, but is faster and uses only 9MB:

PQsendQuery(conn, "select generate_series(1, 10000000)");
PQsetSingleRowMode(conn);
 
int i, value;
PGresult *res;
while (res = PQgetResult(conn)) {
    for (i = 0; i < PQntuples(res); i++) {
        value = atoi(PQgetvalue(res, i, 0));
    }
    PQclear(res);
}

 Some useful links:

https://www.postgresql.org/docs/9.2/static/libpq-single-row-mode.html

https://www.postgresql.org/docs/9.2/static/libpq-async.html