Refactoring: Move locator creating inside BlockChainDownload class.
Refactoring: Move locator creating inside BlockChainDownload class.

This fixes a bug whereby the software would start multiple downloads
concurrently, because the locking would not trigger until after the
block locator was loaded.

bitfloor/bitcoinjs-server,bankonme/bitcoinjs-server,zhangweis/node-jds,dazoe/bitcoinjs-server,dazoe/bitcoinjs-server,javascriptit/bitcoinjs-server,dazoe/bitcoinjs-server,javascriptit/bitcoinjs-server,bitfloor/bitcoinjs-server,zhangweis/node-jds,bankonme/bitcoinjs-server,zhangweis/node-jds,javascriptit/bitcoinjs-server,bankonme/bitcoinjs-server