Attestation processing (#497)

* Renamed fork_choice::process_attestation_from_block

* Processing attestation in fork choice

* Retrieving state from store and checking signature

* Looser check on beacon state validity.

* Cleaned up get_attestation_state

* Expanded fork choice api to provide latest validator message.

* Checking if the an attestation contains a latest message

* Correct process_attestation error handling.

* Copy paste error in comment fixed.

* Tidy ancestor iterators

* Getting attestation slot via helper method

* Refactored attestation creation in test utils

* Revert "Refactored attestation creation in test utils"

This reverts commit 4d277fe4239a7194758b18fb5c00dfe0b8231306.

* Integration tests for free attestation processing

* Implicit conflicts resolved.

* formatting

* Do first pass on Grants code

* Add another attestation processing test

* Tidy attestation processing

* Remove old code fragment

* Add non-compiling half finished changes

* Simplify, fix bugs, add tests for chain iters

* Remove attestation processing from op pool

* Fix bug with fork choice, tidy

* Fix overly restrictive check in fork choice.

* Ensure committee cache is build during attn proc

* Ignore unknown blocks at fork choice

* Various minor fixes

* Make fork choice write lock in to read lock

* Remove unused method

* Tidy comments

* Fix attestation prod. target roots change

* Fix compile error in store iters

* Reject any attestation prior to finalization

* Fix minor PR comments

* Remove duplicated attestation finalization check

* Remove awkward `let` statement

beacon_node/beacon_chain/src/errors.rs

@@ -1,5 +1,8 @@
 use crate::fork_choice::Error as ForkChoiceError;
 use crate::metrics::Error as MetricsError;
+use state_processing::per_block_processing::errors::{
+    AttestationValidationError, IndexedAttestationValidationError,
+};
 use state_processing::BlockProcessingError;
 use state_processing::SlotProcessingError;
 use types::*;
@@ -23,6 +26,7 @@ pub enum BeaconChainError {
         previous_epoch: Epoch,
         new_epoch: Epoch,
     },
+    UnableToFindTargetRoot(Slot),
     BeaconStateError(BeaconStateError),
     DBInconsistent(String),
     DBError(store::Error),
@@ -31,6 +35,11 @@ pub enum BeaconChainError {
     MissingBeaconState(Hash256),
     SlotProcessingError(SlotProcessingError),
     MetricsError(String),
+    NoStateForAttestation {
+        beacon_block_root: Hash256,
+    },
+    AttestationValidationError(AttestationValidationError),
+    IndexedAttestationValidationError(IndexedAttestationValidationError),
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);
@@ -53,3 +62,5 @@ pub enum BlockProductionError {
 easy_from_to!(BlockProcessingError, BlockProductionError);
 easy_from_to!(BeaconStateError, BlockProductionError);
 easy_from_to!(SlotProcessingError, BlockProductionError);
+easy_from_to!(AttestationValidationError, BeaconChainError);
+easy_from_to!(IndexedAttestationValidationError, BeaconChainError);