These methods were named so that they appeared
private by convention. This convention causes
trouble in the documentation where these methods
are righfully dropped. Since these methods aren't
really private in reality (they need to be overwritten
in subclasses), they were renamed, migrating their
naming from `_*` to `do_*`.

This enables the plugin to read the status of the
four formal states a plugin can be in. Previously
even though the four states existed implicitly,
it could only be differentiated between the
STARTED/STOPPED state based on the `running`
bool.

This change also has the nice benefit that we
can make stronger assumptions about the status
inside `start()` and raise an exception if
`start()` was called on a plugin that isn't in
the correct status to call `start()`

Relates to ethereum#1103