fix potential source of problem with some witness_tests

When initially chosen witness happened to be first in next schedule, it would produce despite change in set of witnesses represented by witness plugin (and the block
would be ok, because all witnesses have the same key in those tests). The problem could show in tests where it is expected that most blocks are produced from test
manually with debug plugin.

libraries/plugins/witness/witness_plugin.cpp

@@ -596,6 +596,12 @@ bool witness_plugin::is_fast_confirm_enabled() const
 void witness_plugin::set_witnesses( const std::set< hive::protocol::account_name_type >& witnesses )
 {
   my->_witnesses = witnesses;
+  // recompute next block production data with new set of witnesses
+  {
+    auto data = my->get_produce_block_data( my->_db.get_slot_time( 1 ) );
+    std::unique_lock g( my->should_produce_block_mutex );
+    my->produce_block_data = std::move( data );
+  }
 }
 
 void witness_plugin::set_program_options(