De-duplicate attestations in the slasher (#2767)

## Issue Addressed

Closes https://github.com/sigp/lighthouse/issues/2112
Closes https://github.com/sigp/lighthouse/issues/1861

## Proposed Changes

Collect attestations by validator index in the slasher, and use the magic of reference counting to automatically discard redundant attestations. This results in us storing only 1-2% of the attestations observed when subscribed to all subnets, which carries over to a 50-100x reduction in data stored 🎉 

## Additional Info

There's some nuance to the configuration of the `slot-offset`. It has a profound effect on the effictiveness of de-duplication, see the docs added to the book for an explanation: 5442e695e5/book/src/slasher.md (slot-offset)

slasher/src/attester_record.rs

@@ -1,4 +1,5 @@
 use ssz_derive::{Decode, Encode};
+use std::sync::Arc;
 use tree_hash::TreeHash as _;
 use tree_hash_derive::TreeHash;
 use types::{AggregateSignature, EthSpec, Hash256, IndexedAttestation, VariableList};
@@ -11,6 +12,21 @@ pub struct AttesterRecord {
     pub indexed_attestation_hash: Hash256,
 }
 
+/// Bundling of an `IndexedAttestation` with an `AttesterRecord`.
+///
+/// This struct gets `Arc`d and passed around between each stage of queueing and processing.
+#[derive(Debug)]
+pub struct IndexedAttesterRecord<E: EthSpec> {
+    pub indexed: IndexedAttestation<E>,
+    pub record: AttesterRecord,
+}
+
+impl<E: EthSpec> IndexedAttesterRecord<E> {
+    pub fn new(indexed: IndexedAttestation<E>, record: AttesterRecord) -> Arc<Self> {
+        Arc::new(IndexedAttesterRecord { indexed, record })
+    }
+}
+
 #[derive(Debug, Clone, Encode, Decode, TreeHash)]
 struct IndexedAttestationHeader<T: EthSpec> {
     pub attesting_indices: VariableList<u64, T::MaxValidatorsPerCommittee>,