Initialize execStore while restoring a container from checkpoint.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
test for panic on daemon restart

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Merge pull request #8115 from vishh/exec_panic_fix

Initialize execStore while restoring a container from checkpoint.