Finality notification streams (#791)

* finalization for in_mem

* fetch last finalized block

* pruning: use canonical term instead of final

* finalize blocks in full node

* begin to port light client DB

* add tree-route

* keep number index consistent in full nodes

* fix tests

* disable cache and finish porting light client

* add AsMut to system module

* final leaf is always best

* fix all tests

* Fix comment and trace

* removed unused Into call

* add comment on behavior of `finalize_block`

* move `tree_route` to client common

* tree_route tests

* return slices in TreeRoute

* apply finality up to parent

* add `finalize_block` call

* adjust formatting

* finality notifications and add last finalized block to chain info

* exhaustive match and comments

* fix sync tests by using non-instant finality