* Change `unregister_netdevice` to be an event to fix#47.
* Change `KernelPanic` to `KernelOops` because we can't handle kernel
panic currently.
* Use system boot time instead of "StartPattern" to fix#48.
1) Add lookback support in kernel monitor. After started, Kernel monitor
will check some old logs to detect problems which happened before last
node reboot.
2) Add `lookback` and `startPattern` in kernel monitor configuration.
* `lookback` specifies how long time kernel monitor should look back.
* `startPattern` specifies which log indicates the node is started.
kernel monitor will clear all current node conditions once it finds
a node start log. This makes sure that old problems won't change the
node condition.
3) Add support for kernel panic monitoring, the null pointer and divide
0 kernel panic will be surfaced as event. Usually kernel monitor will
report these events during looking back phase.
