Merge branch 'more-consistent-logging' into master.
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.