Print warnings when fetching pending blocks (#8711)

* Lots of println to figure out what eth_getBlockByNumber does/should do

* Remove debugging

* Print warnings when fetching pending blocks

When calling `eth_getBlockByNumber` with `pending`, we now print a deprecation warning and:

* if a pending block is found, use it to respond
* if no pending block is found, respond as if if was a request for `Latest`

Addresses issue #8703 (not sure if it's enough to close it tbh)

ethcore/src/miner/miner.rs

@@ -290,10 +290,12 @@ impl Miner {
 	{
 		self.sealing.lock().queue
 			.peek_last_ref()
-			.and_then(|b| if b.block().header().number() > latest_block_number {
-				Some(f(b))
-			} else {
-				None
+			.and_then(|b| {
+				if b.block().header().number() > latest_block_number {
+					Some(f(b))
+				} else {
+					None
+				}
 			})
 	}