When reloading we need to disable debug output when the debug plugin is
removed from the config file. Add a `Clear` function to pkg/log and use
it in the server server.

Add test case in pkg/log, for actuall check I manually checked the
output by sprinkling some debug statements in the startup and checking
with sending SIGUSR1.

Also clear up the comments in pkg/log to remove the text about time
stamping.

Fixes: #3035
Signed-off-by: Miek Gieben <miek@miek.nl>