Add slasher broadcast (#2079)

## Issue Addressed

Closes #2048

## Proposed Changes

* Broadcast slashings when the `--slasher-broadcast` flag is provided.
* In the process of implementing this I refactored the slasher service into its own crate so that it could access the network code without creating a circular dependency. I moved the responsibility for putting slashings into the op pool into the service as well, as it makes sense for it to handle the whole slashing lifecycle.

beacon_node/src/cli.rs

@@ -434,6 +434,13 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .requires("slasher")
                 .takes_value(true)
         )
+        .arg(
+            Arg::with_name("slasher-broadcast")
+                .long("slasher-broadcast")
+                .help("Broadcast slashings found by the slasher to the rest of the network \
+                       [disabled by default].")
+                .requires("slasher")
+        )
         .arg(
             Arg::with_name("wss-checkpoint")
                 .long("wss-checkpoint")