Improve the robustness of old ZooKeeper log removal

[trustin]

Motivation:

OldLogRemover in ZooKeeperCommandExecutor currently catches a Throwable when deleting an old log or its log blocks. However, it has two issues doing so:

• It doesn't handle an exception that's raised when reading the metadata of the old log.
• Throwable is way too wide exception to catch. Catching a KeeperException whose code is NONODE will be enough.
  • Note that the failure will only transfer the leadership to other replica, rather than stopping the whole replication process.

Modifications:

• OldLogRemover now catches KeeperException whose code is NONODE only.
• An attempt to read a missing log node's metadata is now handled properly.
• Added more detail to the log messages about missing nodes
  • Split deleteLog() into deleteLog() and deleteLogBlock()

Result:

• The leadership is not transferred anymore when OldLogRemover attempts to retrieve a missing log node's metadata, which is not really a critical issue.
  • Instead, the leadership will be transferred when an exception occurs not because of a missing node.