host: Logbuf: Remove the event system.

The new implementation for tailing will not use the event system because of
possible races, but will keep tailing the logfile itself.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
godep: Update lumberjack to v2.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
host: Logbuf now uses lumberjack v2 and a tailing library.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>