Prune abandoned forks (#916)

* Address compiler warning

* Prune abandoned fork choice forks

* New approach to pruning

* Wrap some block hashes in a newtype pattern

For increased type safety.

* Add Graphviz chain dump emitter for debugging

* Fix broken test case

* Make prunes_abandoned_forks use real DiskStore

* Mark finalized blocks in the GraphViz output

* Refine debug stringification of Slot and Epoch

Before this commit: print!("{:?}", Slot(123)) == "Slot(\n123\n)".
After this commit: print!("{:?", Slot(123)) == "Slot(123)".

* Simplify build_block()

* Rewrite test case using more composable test primitives

* Working rewritten test case

* Tighten fork prunning test checks

* Add another pruning test case

* Bugfix: Finalized blocks weren't always properly detected

* Pruning: Add pruning_does_not_touch_blocks_prior_to_finalization test case

* Tighten pruning tests: check if heads are tracked properly

* Add a failing test case for a buggy scenario

* Change name of function to a more accurate one

* Fix failing test case

* Test case: Were skipped slots' states pruned?

* Style fix: Simplify dereferencing

* Tighten pruning tests: check if abandoned states are deleted

* Towards atomicity of db ops

* Correct typo

* Prune also skipped slots' states

* New logic for handling skipped states

* Make skipped slots test pass

* Post conflict resolution fixes

* Formatting fixes

* Tests passing

* Block hashes in Graphviz node labels

* Removed unused changes

* Fix bug with states having < SlotsPerHistoricalRoot roots

* Consolidate State/BlockRootsIterator for pruning

* Address review feedback

* Fix a bug in pruning tests

* Detach prune_abandoned_forks() from its object

* Move migrate.rs from store to beacon_chain

* Move forks pruning onto a background thread

* Bugfix: Heads weren't pruned when prune set contained only the head

* Rename: freeze_to_state() -> process_finalization()

* Eliminate redundant function parameter

Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/store/src/lib.rs

@@ -15,7 +15,7 @@ pub mod chunked_vector;
 pub mod config;
 mod errors;
 mod forwards_iter;
-mod hot_cold_store;
+pub mod hot_cold_store;
 mod impls;
 mod leveldb_store;
 mod memory_store;
@@ -24,7 +24,6 @@ mod partial_beacon_state;
 mod state_batch;
 
 pub mod iter;
-pub mod migrate;
 
 use std::sync::Arc;
 
@@ -32,7 +31,6 @@ pub use self::config::StoreConfig;
 pub use self::hot_cold_store::{HotColdDB as DiskStore, HotStateSummary};
 pub use self::leveldb_store::LevelDB as SimpleDiskStore;
 pub use self::memory_store::MemoryStore;
-pub use self::migrate::Migrate;
 pub use self::partial_beacon_state::PartialBeaconState;
 pub use errors::Error;
 pub use impls::beacon_state::StorageContainer as BeaconStateStorageContainer;
@@ -132,7 +130,7 @@ pub trait Store<E: EthSpec>: Sync + Send + Sized + 'static {
     }
 
     /// (Optionally) Move all data before the frozen slot to the freezer database.
-    fn freeze_to_state(
+    fn process_finalization(
         _store: Arc<Self>,
         _frozen_head_root: Hash256,
         _frozen_head: &BeaconState<E>,