Chain Selection Subsystem Logic (#3277)

* crate skeleton and type definitions

* add ChainSelectionMessage

* add error type

* run loop

* fix overseer

* simplify determine_new_blocks API

* write an overlay struct and fetch new blocks

* add new function to overlay

* more flow

* add leaves to overlay and add a strong type around leaves-set

* add is_parent_viable

* implement block import, ignoring reversions

* add stagnant-at to overlay

* add stagnant

* add revert consensus log

* flow for reversions

* extract and import block reversions

* recursively update viability

* remove redundant parameter from WriteBlockEntry

* do some removal of viable leaves

* address grumbles

* refactor

* address grumbles

* add comment about non-monotonicity

* extract backend to submodule

* begin the hunt for viable leaves

* viability pivots for updating the active leaves

* remove LeafSearchFrontier

* partially -> explicitly viable and untwist some booleans

* extract tree to submodule

* implement block finality update

* Implement block approval routine

* implement stagnant detection

* ensure blocks pruned on finality are removed from the active leaves set

* write down some planned test cases

* floww

* leaf loading

* implement best_leaf_containing

* write down a few more tests to do

* remove dependence of tree on header

* guide: ChainApiMessage::BlockWeight

* node: BlockWeight ChainAPI

* fix compile issue

* note a few TODOs for the future

* fetch block weight using new BlockWeight ChainAPI

* implement unimplemented

* sort leaves by block number after weight

* remove warnings and add more TODOs

* create test module

* storage for test backend

* wrap inner in mutex

* add write waker query to test backend

* Add OverseerSignal -> FromOverseer conversion

* add test harnes

* add no-op test

* add some more test helpers

* the first test

* more progress on tests

* test two subtrees

* determine-new-blocks: cleaner genesis avoidance and tighter ancestry requests

* don't make ancestry requests when asking for one block

* add a couple more tests

* add to AllMessages in guide

* remove bad spaces from bridge

* compact iterator

* test import with gaps

* more reversion tests

* test finalization pruning subtrees

* fixups

* test clobbering and fix bug in overlay

* exhaustive backend state after finalizaiton tested

* more finality tests

* leaf tests

* test approval

* test ChainSelectionMessage::Leaves thoroughly

* remove TODO

* avoid Ordering::is_ne so CI can build

* comment algorithmic complexity

* Update node/core/chain-selection/src/lib.rs

Co-authored-by: Bernhard Schuster <[email protected]>

Co-authored-by: Bernhard Schuster <[email protected]>