plugin: Add new notification type: warning

This notification bases on `LOG_BROKEN` and `LOG_UNUSUAL` level log.

--Introduction

A notification for topic `warning` is sent every time a new `BROKEN`/
`UNUSUAL` level(in plugins, we use `error`/`warn`) log generated, which
 means an unusual/borken thing happens, such as channel failed,
message resolving failed...

```json
{
	"warning": {
	"level": "warn",
	"time": "1559743608.565342521",
	"source": "lightningd(17652): 0821f80652fb840239df8dc99205792bba2e559a05469915804c08420230e23c7c chan #7854:",
	"log": "Peer permanent failure in CHANNELD_NORMAL: lightning_channeld: sent ERROR bad reestablish dataloss msg"
  }
}
```
1. `level` is `warn` or `error`:
`warn` means something seems bad happened and it's under control, but
we'd better check it;
`error` means something extremely bad is out of control, and it may lead
to crash;

2. `time` is the second since epoch;

3. `source`, in fact, is the `prefix` of the log_entry. It means where
the event happened, it may have the following forms:
`<node_id> chan #<db_id_of_channel>:`, `lightningd(<lightningd_pid>):`,
`plugin-<plugin_name>:`, `<daemon_name>(<daemon_pid>):`, `jsonrpc:`,
`jcon fd <error_fd_to_jsonrpc>:`, `plugin-manager`;

4. `log` is the context of the original log entry.

--Note:

1. The main code uses `UNUSUAL`/`BROKEN`, and plugin module uses `warn`
/`error`, considering the consistency with plugin, warning choose `warn`
/`error`. But users who use c-lightning with plugins may want to
`getlog` with specified level when receive warning. It's the duty for
plugin dev to turn `warn`/`error` into `UNUSUAL`/`BROKEN` and present it
 to the users, or pass it directly to `getlog`;

2. About time, `json_log()` in `log` module uses the Relative Time, from
 the time when `log_book` inited to the time when this event happend.
 But I consider the `UNUSUAL`/`BROKEN` event is rare, and it is very
 likely to happen after running for a long time, so for users, they will
  pay more attention to Absolute Time.

-- Related Change

1. Remove the definitions of `log`, `log_book`, `log_entry` from `log.c`
to `log.h`, then they can be used in warning declaration and definition.

2. Remove `void json_add_time(struct json_stream *result, const char
*fieldname, struct timespec ts)` from `log.c` to `json.c`, and add
related declaration in `json.h`. Now the notification function in
`notification.c` can call it.

2. Add a pointer to `struct lightningd` in `struct log_book`. This may
affect the independence of the `log` module, but storing a pointer to
`ld` is more direct;

lightningd/json.c

@@ -429,3 +429,14 @@ void json_add_timeabs(struct json_stream *result, const char *fieldname,
 	json_add_member(result, fieldname, "%" PRIu64 ".%03" PRIu64,
 			(u64)t.ts.tv_sec, (u64)t.ts.tv_nsec / 1000000);
 }
+
+void json_add_time(struct json_stream *result, const char *fieldname,
+			  struct timespec ts)
+{
+	char timebuf[100];
+
+	snprintf(timebuf, sizeof(timebuf), "%lu.%09u",
+		(unsigned long)ts.tv_sec,
+		(unsigned)ts.tv_nsec);
+	json_add_string(result, fieldname, timebuf);
+}