core, eth, internal: Added debug_getBadBlocks() method (#3654)

* core,eth,internal: Added `debug_getBadBlocks()` method

When bad blocks are discovered, these are stored within geth.
An RPC-endpoint makes them availablewithin the `debug`
namespace. This feature makes it easier to discover network forks.

```

* core, api: go format + docs

* core/blockchain: Documentation, fix minor nitpick

* core: fix failing blockchain test

core/blockchain_test.go

@@ -485,6 +485,7 @@ func chm(genesis *types.Block, db ethdb.Database) *BlockChain {
 	bc.bodyRLPCache, _ = lru.New(100)
 	bc.blockCache, _ = lru.New(100)
 	bc.futureBlocks, _ = lru.New(100)
+	bc.badBlocks, _ = lru.New(10)
 	bc.SetValidator(bproc{})
 	bc.SetProcessor(bproc{})
 	bc.ResetWithGenesisBlock(genesis)