Atomic operation and locking when importing a block (#1489)

* Add ClientImportOperation and remove an unused enum

* set_aux to insert_aux so that it can be called multiple times

* [WIP] All basic designs of lock_import_and_run

* [WIP] `apply_block` and `apply_aux` implementation

* Update client db with the new interface

* Always make sure we reset importing_block back to None

* Address grumbles

`apply_block` should be pub

* Add comments on insert_aux

* Fix compile