Shutdown after sync (#2519)

## Issue Addressed

Resolves #2033 

## Proposed Changes

Adds a flag to enable shutting down beacon node right after sync is completed.

## Additional Info

Will need modification after weak subjectivity sync is enabled to change definition of a fully synced node.

beacon_node/src/cli.rs

@@ -51,6 +51,12 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .help("Disables the discovery packet filter. Useful for testing in smaller networks")
                 .takes_value(false),
         )
+        .arg(
+            Arg::with_name("shutdown-after-sync")
+                .long("shutdown-after-sync")
+                .help("Shutdown beacon node as soon as sync is completed")
+                .takes_value(false),
+        )
         .arg(
             Arg::with_name("zero-ports")
                 .long("zero-ports")