Rollup of various testnet-related fixes.

fix a deadlock when spawning agreement as non-authority

fix test compilation for BFT

more accurate consensus superseding logic

mild revision to `can_build_on` logic

block evaluation without redundant initialisation

refactor BFT delay: update rhododendron and poll after delaying. (#589)

dropping BFT future before poll doesn't lead to service deadlock

polkadot/api/src/lib.rs

@@ -32,6 +32,9 @@ extern crate substrate_state_machine as state_machine;
 #[macro_use]
 extern crate error_chain;
 
+#[macro_use]
+extern crate log;
+
 #[cfg(test)]
 extern crate substrate_keyring as keyring;