AppendArr is not implemented everywhere

[arnaudbriche]

I have a used case where I must insert in batch in a wide table with more than 100 columns and about 40 array columns.
The data is coming in small batches of a few hundreds rows and is appended to a ColInput until there is enough rows to send to the server.
I benchmarked a bit and find out that Appending new data to ColInputs was quite expensive, sometimes taking 10s of milliseconds.
I modified the code a bit to ensure AppendArr method was usable on every ColInput, and find out that it lead to a 10-fold improvement in the time taken to append new batches.
I'm preparing a PR that implements this change.