Implement freezer database (#508)

* Implement freezer database for state vectors

* Improve BeaconState safe accessors

And fix a bug in the compact committees accessor.

* Banish dodgy type bounds back to gRPC

* Clean up

* Switch to exclusive end points in chunked vec

* Cleaning up and start of tests

* Randao fix, more tests

* Fix unsightly hack

* Resolve test FIXMEs

* Config file support

* More clean-ups, migrator beginnings

* Finish migrator, integrate into BeaconChain

* Fixups

* Fix store tests

* Fix BeaconChain tests

* Fix LMD GHOST tests

* Address review comments, delete 'static bounds

* Cargo format

* Address review comments

* Fix LMD ghost tests

* Update to spec v0.9.0

* Update to v0.9.1

* Bump spec tags for v0.9.1

* Formatting, fix CI failures

* Resolve accidental KeyPair merge conflict

* Document new BeaconState functions

* Fix incorrect cache drops in `advance_caches`

* Update fork choice for v0.9.1

* Clean up some FIXMEs

* Fix a few docs/logs

* Update for new builder paradigm, spec changes

* Freezer DB integration into BeaconNode

* Cleaning up

* This works, clean it up

* Cleanups

* Fix and improve store tests

* Refine store test

* Delete unused beacon_chain_builder.rs

* Fix CLI

* Store state at split slot in hot database

* Make fork choice lookup fast again

* Store freezer DB split slot in the database

* Handle potential div by 0 in chunked_vector

* Exclude committee caches from freezer DB

* Remove FIXME about long-running test

beacon_node/store/src/impls/partial_beacon_state.rs

@@ -0,0 +1,16 @@
+use crate::*;
+use ssz::{Decode, Encode};
+
+impl<T: EthSpec> SimpleStoreItem for PartialBeaconState<T> {
+    fn db_column() -> DBColumn {
+        DBColumn::BeaconState
+    }
+
+    fn as_store_bytes(&self) -> Vec<u8> {
+        self.as_ssz_bytes()
+    }
+
+    fn from_store_bytes(bytes: &[u8]) -> Result<Self, Error> {
+        Ok(Self::from_ssz_bytes(bytes)?)
+    }
+}