instead of conf and args, so that it's easier to test the method
without creating ConfigParser and ArgumentParser objects.

1. magic methods
2. classmethods
3. staticmethods
4. properties
5. methods

1. magic methods
2. classmethods
3. staticmethods
4. properties
5. methods

1. magic methods
2. classmethods
3. staticmethods
4. properties
5. methods

teor authored Jul 18, 2018 and juga committed Jul 23, 2018

Also:
* document the location of latest.v3bw
* document the atomic creation of latest.v3bw
* explain how to atomically create latest.v3bw when transferring it to
  another host

Closes tor trac 26740.

- Remove started_at file in config et al.
- Add state_fname to config et al.
- Remove read/write functions for the started_at file in scanner.py
  and genearte.py

but not allowing /dev/stdout

This required a bump in the result version: we add a new field to
ResultSuccess.

It doesn't need to know how to write to a file. It store and organizes
data.

I think this is the more common way to do things. It allows you to move
the files around without the symlink breaking.

also fix link path

also lock the directory except when writing to the standard output,
needed for test_generate.py

by mistake from_results was calling latest_bandwidth instead of
earliest_bandwidth

to make it less confussing.
Note that stem call it ed25519_master_key instead of
master_key_ed25519, but dir-spec.txt uses master-key-ed25519, which
is the term chosen here.

also, convert types to int when generating line from str

also complete classes docstring

* Move logic in generate to class
* Add test
* Move 7500 to global constant
* Make read_started_at not fail when conf is None

    * Move V3BWLine to v3bwfile.py
    * Add methods to V3BWLine similar to V3BwHeader
    * Add methods to V3BWLine to implement most of the logic in the class
    * Move functions related to data lines to v3bwfile.py
    * Add functions to generate types of erros
    * Add tests for the class and conftest to read results from file
    * Fix other tests depending on bw lines

so that it close to the header and we can reuse methods/functions

also:
* remove latest_bandwidth outside of class, since it should be
  generated from timestamp
* pass all init arguments as string
* require timestamp argument
* fix tests with changes

they do not need to be in 3rd and 4th posstions.