Reorder paragraphs (signing first, verification after), mention hash-by-hash
comparison of injections.

With this, the only work pending on the whitepaper is describing the
multi-peer download protocol (still marked as TODOv6).

And fix references to the use of Base64 encoding in chunk extensions.

It is needed besides the previous block's chained hash (`ouihash`) for the
first data block's signature string.

Yes, chained hashes lose the property of being able to be used for the
comparison of "prefix blocks" between different injections (as the injection
id is included in the hashing).  That responsibility has moved to the
multi-peer download protocol instead, by downloading all data hashes of blocks
beforehand and comparing them literally.

This release makes logging more consistent (especially at the client) and
offers some goodies for testing and debugging the client from its front-end.

Features:

  - Get the list of groups announced by the client from its front-end
    (`/groups.txt`).
  - Toggle log file creation from the client front-end, provide a download
    link.
  - Enabling the log file enables debugging and restores the previous log
    level when disabled, unless the user explicitly changes it in the
    meanwhile.

Enhancements:

  - More consistent logging: always use the logger (i.e. show DEBUG, WARN,
    ERROR, INFO tags), fix the log level of many messages (e.g. show requests
    and responses as DEBUG when debugging.
  - Use a single log level for all components (i.e. drop the separate log
    level for the BEP5/HTTP cache).

Bugfixes:

  - Fix content being announced not getting reannounced if reinjected shortly
    after purging the local cache, when not debugging.
  - Fix BEP5 swarm status not being reported when enabling debugging at
    runtime.

This fixes several inconsistencies in logging, particularly:

  - Respect log level even when log file is on.
  - Enabling the log file automatically enables debugging, disabling the file
    restores the previous log level.  The log level can be changed by the user
    explicitly in the meanwhile: log messages honor this setting and the old
    level is not restored in this case.
  - The log file can be toggled and downloaded from the client front-end.
  - Single log level also affecting BEP5/HTTP cache code.
  - BEP5 swarm status were not being shown unless started with debug.
  - Some messages were sent directly to stderr instead of via the logger.
  - Some debugging messages were shown as INFO (esp. request/response heads).
  - Some debugging messages from the same module were missing a consistent
    prefix.

It also fixes a bug which caused content being announced not to get
reannounced if reinjected shortly after purging the local cache, when not
debugging.

The user should not usually be interested in internal coroutines, at least
unless they refuse to stop.

Also show error code in some pending cases.

To avoid boilerplate in messages logged via `Yield`, also for abort message on
exception at top level.

This also fixes commit b583c0e1 in case the client has debugging off.

A common prefix appears in all plain messages, and no prefix (just the tag)
appears in messages logged via `Yield`.

If missing, stay with `INFO` for backwards compatibility.

Also, detect early when the given level is disabled, and avoid doing string
conversion and line splitting.

And remove one unused such function.

Same as with client, this is handled elsewhere (client front-end).

And setting it from client front-end.  The announcer still needs to get its
previously inherited log level setting removed.

This is handled elsewhere (client front-end), the client should not debug if
debugging is off, regardless of whether logging to a file or not.

There has not been information worth refreshing on the front-end for a while,
and it makes using its controls quite tricky.

With download link.

Recover the log level once the log file is disabled (unless explicitly set by
the user while logging to the file).

By copying the key which may become trashed on cancellation (when debugging).

So that it does produce output if the log level enables debugging during
runtime.

This adds a link to the client front-end to retrieve a list of groups being
announced by this client.  Directly available as `/groups.txt` from the
front-end.

To avid getting in the middle of other text.

As plain text, one per line, undefined ordering.

This release fixes some build errors in Android and a wrong assertion which
may interfere with debug builds.

Fix some issues with the Android build.

It triggers an error in the Android build.