Improve performance of CURRENT_TIME

[asfernandes]

As conversion from TIMESTAMP to TIME (both WITH TIME ZONE) is not a trivial operation, caching its converted value in the request makes ~15x improvement in its performance in my tests.

Test case:

execute block
as
    declare n integer = 1;
    declare d time with time zone;
begin
    while (n < 1000000)
    do
    begin
        n = n + 1;
        d = current_time;
    end
end!