Logger: rename getWriter() -> createWriter()

refs #7060
Logger: implement getWriter() and getInstance()

refs #7060
FileWriter: implement getPath()

refs #7060
Application log: don't crash if logData === null

refs #7060
Application log: don't use Icinga\Application\Config to get the log file name

refs #7060
Logger: add/update documentation

refs #7060
Foreach: add $value so that the value won't be assigned to $key

refs #7060
Icinga\Protocol\File\Reader::fetchPairs(): rename variables

$skipLines -> $skip
$readLines -> $read

refs #7060
Icinga\Protocol\File\Reader::fetchPairs(): increase $index manually as the iterator's one makes trouble

refs #7060
Merge branch 'master' into bugfix/rebuild-application-log-logic-7060
Merge branch 'bugfix/rebuild-application-log-logic-7060'

Don't work on this branch for this issue because the application log logic has nothing to do with #7060.
(Work on application log related improvements will continue on another branch.)