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.

slasher/src/batch_stats.rs

@@ -0,0 +1,16 @@
+#[derive(Debug)]
+pub struct BatchStats {
+    pub block_stats: BlockStats,
+    pub attestation_stats: AttestationStats,
+}
+
+#[derive(Debug)]
+pub struct BlockStats {
+    pub num_processed: usize,
+    pub num_slashings: usize,
+}
+
+#[derive(Debug)]
+pub struct AttestationStats {
+    pub num_processed: usize,
+}