Revert "block: Add API for urgent request handling"

This reverts commit 75771402e2aad6509ee855b5135949668e0f7c60.
Revert "Preapartion for new API"

This reverts commit fcdc52ecbde5432a18d29d6c311d1f08850a9829.
Preparation for new insertion
block: Add support for reinsert a dispatched req

Add support for reinserting a dispatched request back to the
scheduler's internal data structures.
This capability is used by the device driver when it chooses to
interrupt the current request transmission and execute another (more
urgent) pending request. For example: interrupting long write in order
to handle pending read. The device driver re-inserts the
remaining write request back to the scheduler, to be rescheduled
for transmission later on.

Add API for verifying whether the current scheduler
supports reinserting requests mechanism. If reinsert mechanism isn't
supported by the scheduler, this code path will never be activated.
Revert "Revert "block: Add API for urgent request handling""

This reverts commit a8845e666e18c7fefb90da4a04f8c2ae7c46bb3f.
block: elevator: Add missing function for new API