SafeMode should count only complete blocks.

During start up the name-node is in safe mode and is counting blocks reported by data-nodes. When the number of minimally replicated blocks reaches the configured threshold the name-node leaves safe mode. Currently all blocks are counted towards the threshold including the ones that are under construction. The under-construction blocks should be excluded from the count, because they need to be recovered, which may take long time (lease expires in 1 hour by default). Also the recovery may result in deleting those blocks so counting them in the blocks total is incorrect.