Each TDF node keeps a thread-local tuple of TColumnValues. TColumnValues
read the values of a column for a certain entry, but entry ranges and
TTreeReader-related objects are _task_ local.

Before this change, in case of interleaved task execution, the second
task starting in a thread would re-initialize the TColumnValues and
 overwrite the previous task's settings.

Now TColumnValues deal with multiple initializations by keeping their
internal state in a stack: when a new task initializes a TColumnValue
it pushes the required values in the stack, when a task completes
execution it pops its values from the stack.

Note that interleaved TBB tasks always follow a last-to-start,
first-to-finish pattern, which allows us to use a LIFO structure for
TColumnValue.