interfaces: add interruptWait method

- This method can be used to cancel a running
  waitNext().

- This commit also adds a test case for interruptWait method

src/ipc/capnp/mining.capnp

@@ -33,6 +33,7 @@ interface BlockTemplate $Proxy.wrap("interfaces::BlockTemplate") {
     getCoinbaseMerklePath @8 (context: Proxy.Context) -> (result: List(Data));
     submitSolution @9 (context: Proxy.Context, version: UInt32, timestamp: UInt32, nonce: UInt32, coinbase :Data) -> (result: Bool);
     waitNext @10 (context: Proxy.Context, options: BlockWaitOptions) -> (result: BlockTemplate);
+    interruptWait @11() -> ();
 }
 
 struct BlockCreateOptions $Proxy.wrap("node::BlockCreateOptions") {