docs: replace explanation on how to store the measurements

Source code: :func:`sbws.core.scanner._should_keep_result`

Simple result storage

~~~~~~~~~~~~~~~~~~~~~

Writing the measurements to the filesystem

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Internally, sbws has a hierarchy of ``Result`` classes for easy managing of

different types of result (success, error-because-of-circuit-error,

error-because-[...] etc.). These results get converted into JSON strings and

stored -- **one per line** -- in text files in a data directory.

For every measured relay, the measurement result is put in a queue.

There's an independent thread getting measurements from the queue every second.

Every new measurement is appended to a file as a json line

(but the file itself is not json!).

The file is named with the current date. Every day a new file is created.

The text files are simply named after the date. For example:

``2018-03-20.txt``.

The sbws scanner only appends to these files, and it automatically starts a new

file when the system's clock ticks past midnight UTC.

To avoid any weird timezone-related issues, consumers of sbws scanner data (such

as the generate and stats scripts) SHOULD read more files than strictly

necessary. For example, if the validity period is 5 days, they should read 6

days of files. Because all results have a Unix timestamp, consumers of sbws

data can easily determine which results are just outside the validity period as

they are reading them in.

Simple result processing

~~~~~~~~~~~~~~~~~~~~~~~~

Source code: :func:`sbws.lib.resultdump.ResultDump.enter`

Every hour the directory authorities vote to come to a consensus about the

state of the Tor network.  The bandwidth authorities need to use the results