lower the noise for randomwalk tests

Randomwalk test output at the WARN and ERROR levels are pretty good. We are now getting fewer failures and its getting harder to find them among the spurious messages we get:

• zookeeper InterruptedException, which is logged at WARN, but is not an error
• the hangup detector is firing on nodes that take a long time: bulk.Verify and shard.VerifyIndex
• bulk imports need to retry longer after they are farmed out for processing