Simplify IOPlugin.Data initialization

[zenhack]

Issue by Lekensteyn
Friday Jan 08, 2016 at 00:01 GMT
Originally opened as kuba/simp_le#63

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Remove the need to specify every component of Data. This introduces a
new IOPlugin.PersistedData member which is the same as IOPlugin.Data,
except that it is initialized with boolean False instead of None.

Also remove account_key and key from FullChainFile, it is not stored by
ChainFile. Replace a weird IOPlugin.EMPTY_DATA._replace(key=X) hack by
the more intuitive Data(key=X).

The __defaults__ construct is taken from
http://stackoverflow.com/a/18348004 and documented in
https://docs.python.org/2/reference/datamodel.html and
https://docs.python.org/3/reference/datamodel.html

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Lekensteyn included the following code: https://github.com/kuba/simp_le/pull/63/commits