eventq: add a function for replacing head of the queue

eventq: add a function for replacing head of the queue
This patch adds a function for replacing the event at the head of the queue
with another event. This helps in running a different set of events. Events
already scheduled can processed by replacing the original head event back.
This function has been specifically added to support cache warmup and
cooldown required for creating and restoring checkpoints.

--HG--
extra : rebase_source : ed6e2905720b6bfdefd020fab76235ccf33d28d1