Don't deform column-by-column in composite_to_json

Hi,

In https://postgr.es/m/20190201162404.onngi77f26baem4g%40alap3.anarazel.de
I noticed that composite_to_json() deforms column-by-column. Given that
it always processes all columns, that seems quite the waste of resources.

In some quick'n dirty dirty testing this gives a ~4% benefit in a table
without nulls and varlenas, and ~7% in one with both. I assume that if
one were to test with a bit wider table the win would be bigger.

A short test shows that it'd be slower to allocate nulls/values with
palloc rather than using MaxHeapAttributeNumber. Given that only output
functions are called from within composite_to_json(), I think that's ok.

Greetings,

Andres Freund