Add ability to populate missing blocks

* The command populates up to the highest known block number
 * The anticipated use case is that the listener will be running
   in parallel to the populateBlocks command
    * This will mean that the listener is responsible for picking up
      new blocks, and the populateBlocks command is reposible for
      historical blocks
 * Reformat SQL statements

pkg/core/blockchain.go

@@ -1,6 +1,7 @@
 package core
 
 type Blockchain interface {
+	GetBlockByNumber(blockNumber int64) Block
 	SubscribeToBlocks(blocks chan Block)
 	StartListening()
 	StopListening()