Issue #90: Improve addblock logging by reporting every 10 seconds instead of X number of blocks

blockImporter now has these new properties:

txProcessed - number of transactions processed by the importer
lastHeight - height of the last imported block
lastBlockTime - block time of the last imported block
lastLogTime - last timestamp of when a progress message was shown

the p/progress option in addblocks now indicates how often you want the progress message to appear