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)

lighthouse/tests/beacon_node.rs

@@ -795,6 +795,25 @@ fn slasher_update_period_flag() {
         });
 }
 #[test]
+fn slasher_slot_offset() {
+    // TODO: check that the offset is actually stored, once the config is un-hacked
+    // See: https://github.com/sigp/lighthouse/pull/2767#discussion_r741610402
+    CommandLineTest::new()
+        .flag("slasher", None)
+        .flag("slasher-max-db-size", Some("16"))
+        .flag("slasher-slot-offset", Some("11.25"))
+        .run();
+}
+#[test]
+#[should_panic]
+fn slasher_slot_offset_nan() {
+    CommandLineTest::new()
+        .flag("slasher", None)
+        .flag("slasher-max-db-size", Some("16"))
+        .flag("slasher-slot-offset", Some("NaN"))
+        .run();
+}
+#[test]
 fn slasher_history_length_flag() {
     CommandLineTest::new()
         .flag("slasher", None)