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>

validator_client/src/cli.rs

@@ -94,6 +94,13 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                       node is not synced.",
                 ),
         )
+        .arg(
+            Arg::with_name("use-long-timeouts")
+                .long("use-long-timeouts")
+                .help("If present, the validator client will use longer timeouts for requests \
+                        made to the beacon node. This flag is generally not recommended, \
+                        longer timeouts can cause missed duties when fallbacks are used.")
+        )
         // This overwrites the graffiti configured in the beacon node.
         .arg(
             Arg::with_name("graffiti")