discuss multiple call to set_next_input

See https://github.com/ipython/ipykernel/pull/323,

before big split, we were making sure each execution requests would not trigger multiple `set_next_input`, that was due to `?` triggering multiple `set_next_input` in a for loop and annoying some users.

I can see how calling multiple `set_next_input` could make sens, but I don't think the protection code should be in the kernel.