Rename/rethink AttemptId in DirMgr

For #803 (closed) I want to log AttemptId values to help trace which directory success/failure is which.

But maybe we need a better name. Currently, AttemptId is an identifier for a long series of attempts to do different things, starting from zero. From the docs:

/// Identifier for an attempt to bootstrap a directory.
///
/// Every time that we decide to download a new directory, _despite already
/// having one_, counts as a new attempt.
///
/// These are used to track the progress of each attempt independently.

So maybe this needs a different name than "attempt".