Reduce outbound requests to eth1 endpoints (#2340)

## Issue Addressed

#2282 

## Proposed Changes

Reduce the outbound requests made to eth1 endpoints by caching the results from `eth_chainId` and `net_version`.
Further reduce the overall request count by increasing `auto_update_interval_millis` from `7_000` (7 seconds) to `60_000` (1 minute). 
This will result in a reduction from ~2000 requests per hour to 360 requests per hour (during normal operation). A reduction of 82%.

## Additional Info

If an endpoint fails, its state is dropped from the cache and the `eth_chainId` and `net_version` calls will be made for that endpoint again during the regular update cycle (once per minute) until it is back online.


Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/eth1/src/inner.rs

@@ -2,10 +2,12 @@ use crate::Config;
 use crate::{
     block_cache::{BlockCache, Eth1Block},
     deposit_cache::{DepositCache, SszDepositCache},
+    service::EndpointsCache,
 };
 use parking_lot::RwLock;
 use ssz::{Decode, Encode};
 use ssz_derive::{Decode, Encode};
+use std::sync::Arc;
 use types::ChainSpec;
 
 #[derive(Default)]
@@ -28,6 +30,7 @@ impl DepositUpdater {
 pub struct Inner {
     pub block_cache: RwLock<BlockCache>,
     pub deposit_cache: RwLock<DepositUpdater>,
+    pub endpoints_cache: RwLock<Option<Arc<EndpointsCache>>>,
     pub config: RwLock<Config>,
     pub remote_head_block: RwLock<Option<Eth1Block>>,
     pub spec: ChainSpec,
@@ -87,6 +90,7 @@ impl SszEth1Cache {
                 cache: self.deposit_cache.to_deposit_cache()?,
                 last_processed_block: self.last_processed_block,
             }),
+            endpoints_cache: RwLock::new(None),
             // Set the remote head_block zero when creating a new instance. We only care about
             // present and future eth1 nodes.
             remote_head_block: RwLock::new(None),