Reduce log level when block miner is interrupted

[ajsutton]

PR description

Often during shutdown the miner prints:

2019-02-28 07:05:53.717+10:00 | pool-9-thread-1 | ERROR | BlockMiner | Block mining was interrupted.
java.lang.InterruptedException: sleep interrupted
	at java.lang.Thread.sleep(Native Method) ~[?:1.8.0_172]
	at tech.pegasys.pantheon.ethereum.blockcreation.AbstractBlockScheduler.waitUntilNextBlockCanBeMined(AbstractBlockScheduler.java:31) ~[classes/:?]
	at tech.pegasys.pantheon.ethereum.blockcreation.BlockMiner.mineBlock(BlockMiner.java:90) ~[classes/:?]
	at tech.pegasys.pantheon.ethereum.blockcreation.BlockMiner.run(BlockMiner.java:72) [classes/:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_172]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_172]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_172]

This PR fixes the interrupt handling so the log is at debug level, the thread continues to be marked as interrupted and BlockMiner doesn't start creating a new block if it's already been cancelled.