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)

beacon_node/src/cli.rs

@@ -451,6 +451,18 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .requires("slasher")
                 .takes_value(true)
         )
+        .arg(
+            Arg::with_name("slasher-slot-offset")
+                .long("slasher-slot-offset")
+                .help(
+                    "Set the delay from the start of the slot at which the slasher should ingest \
+                     attestations. Only effective if the slasher-update-period is a multiple of the \
+                     slot duration."
+                )
+                .value_name("SECONDS")
+                .requires("slasher")
+                .takes_value(true)
+        )
         .arg(
             Arg::with_name("slasher-history-length")
                 .long("slasher-history-length")