Jumbo chunk flag isn't automatically cleared after a chunk split

Regading the MongoDB documentation about jumbo chunks, MongoDB removes the jumbo flag upon successful split of the chunk. That's not the actual behaviour, though.

The current behaviour is that after a successful split of a jumbo chunk we get 2 new jumbo chunks because new chunks are initialised as a copy of the original chunk.

Since there is no test controlling that, the scope of this ticket should also include a test case to cover that scenario.
You will find attached to this ticket a brand new test file to force the issue. It must be run on the sharding suite.