Fix a bug, which involved clarifying the code.
Fix a bug, which involved clarifying the code.

When AsyncLogClient::GetEntries returned a non-OK status, we would
return without calling WalkEntries. If this range fetch happened to be
the only one, that meant that the overall operation would get hung. I
converted the range fetching to be its own util::Task-driven operation,
ensuring that WalkEntries would *always* get called, no matter what,
and allowing me to remove the manual adding/removing of holds on the
task (since child tasks have an implicit hold), making it one less
thing to get wrong.

I also renamed some methods for better clarity and searchability.

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