Adjust beacon node timeouts for validator client HTTP requests (#2352)

## Issue Addressed

Resolves #2313 

## Proposed Changes

Provide `BeaconNodeHttpClient` with a dedicated `Timeouts` struct.
This will allow granular adjustment of the timeout duration for different calls made from the VC to the BN. These can either be a constant value, or as a ratio of the slot duration.

Improve timeout performance by using these adjusted timeout duration's only whenever a fallback endpoint is available.

Add a CLI flag called `use-long-timeouts` to revert to the old behavior.

## Additional Info

Additionally set the default `BeaconNodeHttpClient` timeouts to the be the slot duration of the network, rather than a constant 12 seconds. This will allow it to adjust to different network specifications.


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

account_manager/src/validator/exit.rs

@@ -4,7 +4,7 @@ use clap::{App, Arg, ArgMatches};
 use environment::Environment;
 use eth2::{
     types::{GenesisData, StateId, ValidatorData, ValidatorId, ValidatorStatus},
-    BeaconNodeHttpClient,
+    BeaconNodeHttpClient, Timeouts,
 };
 use eth2_keystore::Keystore;
 use eth2_network_config::Eth2NetworkConfig;
@@ -81,6 +81,7 @@ pub fn cli_run<E: EthSpec>(matches: &ArgMatches, env: Environment<E>) -> Result<
     let client = BeaconNodeHttpClient::new(
         SensitiveUrl::parse(&server_url)
             .map_err(|e| format!("Failed to parse beacon http server: {:?}", e))?,
+        Timeouts::set_all(Duration::from_secs(env.eth2_config.spec.seconds_per_slot)),
     );
 
     let testnet_config = env