Add flag to import all attestations (#1941)

## Issue Addressed

NA

## Proposed Changes

Adds the `--import-all-attestations` flag which tells the `network::AttestationService` to import/aggregate all attestations after verification (instead of only ones for subnets that are relevant to local validators).

This is useful for testing/debugging and also for creating back-up nodes that should be all cached up and ready for any validator.

## Additional Info

NA

beacon_node/src/cli.rs

@@ -37,6 +37,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                        This will also advertise the beacon node as being long-lived subscribed to all subnets.")
                 .takes_value(false),
         )
+        .arg(
+            Arg::with_name("import-all-attestations")
+                .long("import-all-attestations")
+                .help("Import and aggregate all attestations, regardless of validator subscriptions. \
+                       This will only import attestations from already-subscribed subnets, use with \
+                       --subscribe-all-subnets to ensure all attestations are received for import.")
+                .takes_value(false),
+        )
         .arg(
             Arg::with_name("zero-ports")
                 .long("zero-ports")

beacon_node/src/config.rs

@@ -373,6 +373,10 @@ pub fn set_network_config(
         config.subscribe_all_subnets = true;
     }
 
+    if cli_args.is_present("import-all-attestations") {
+        config.import_all_attestations = true;
+    }
+
     if let Some(listen_address_str) = cli_args.value_of("listen-address") {
         let listen_address = listen_address_str
             .parse()