What is the dead time in between two subsequent outputs in the CD4017 counter?
The Timing Diagram in the Datasheet shows there is a slight delay as each output changes, so Q1 will be high for a fraction of a second before Q0 is low. In practical terms they are instantaneously done. 



From the Datasheet, the longest possible Low to High or High to Low transition time is 650 nano seconds @ 5V.