EPE-389 net_plugin stall during head catchup

This fix handles situations where a node is in head_catchup state and
recieves the sync_last_requested_num and then does a sync_update_expected
for the next block. And the next block has already been received by the
node. In this instance, the block will not receive the next block and
will be effectively hung.

plugins/net_plugin/net_plugin.cpp

@@ -147,7 +147,7 @@ namespace eosio {
 
    private:
       constexpr static auto stage_str( stages s );
-      void set_state( stages s );
+      bool set_state( stages s );
       bool is_sync_required( uint32_t fork_head_block_num );
       void request_next_chunk( std::unique_lock<std::mutex> g_sync, const connection_ptr& conn = connection_ptr() );
       void start_sync( const connection_ptr& c, uint32_t target );
@@ -1372,12 +1372,13 @@ namespace eosio {
     }
   }
 
-   void sync_manager::set_state(stages newstate) {
+   bool sync_manager::set_state(stages newstate) {
       if( sync_state == newstate ) {
-         return;
+         return false;
       }
       fc_ilog( logger, "old state ${os} becoming ${ns}", ("os", stage_str( sync_state ))( "ns", stage_str( newstate ) ) );
       sync_state = newstate;
+      return true;
    }
 
    void sync_manager::sync_reset_lib_num(const connection_ptr& c) {
@@ -1808,7 +1809,9 @@ namespace eosio {
          } );
 
          if( set_state_to_head_catchup ) {
-            set_state( head_catchup );
+            if( set_state( head_catchup ) ) {
+                send_handshakes();
+            }
          } else {
             set_state( in_sync );
             send_handshakes();