txm: change tuple ownership strategy

Since a space holds pointers to tuples it must increment
reference counters of its tuples and must decrement counters
of tuples that are deleted from the space.

Memtx TX manager also holds references to processing tuples
in the same way.

Before this patch there was a logic: while a tuple is dirty
it belongs to TX manager and does not belong to the space. Only
when a tuple is cleared and it is still in space it will be
referenced by space.

That logic leads to crashes in some DDL requests since they
works with indexes directly. For example deleting an index
causes dereferencing of all its tuples - even dirty.

This patch changes the logic. Now all tuples that are physically
in the primary index of the space a referenced. Once removed from
primary index - the tuple is dereferenced. TX manager references
tuples like before - every holding tuple is referenced.

Part of #5628