pgx Module: Add support for handling query result sets with a passed func

[nickman]

Enhancement to the pgx module where query returned rows can be handled by a passed func rather than returning the whole result set.

e.g.

p := pgx.connect("postgres://user:[email protected]:5432/db")
rcnt := p.queryFunc("SELECT * FROM generate_series($1::integer, $2::integer)", func(row) {
    return true
}, 1, 100000)
printf("rows returned: %d\n", rcnt)
p.close()

Some benefits when:

• Processing large datasets, as it allows for processing each row as it is received from the database, rather than waiting for the entire result set to be returned.
• When result set processing should be stopped based on a condition.

Updated docs and provided unit tests.