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/log.c

@@ -4,7 +4,6 @@
 #include <ccan/array_size/array_size.h>
 #include <ccan/err/err.h>
 #include <ccan/io/io.h>
-#include <ccan/list/list.h>
 #include <ccan/opt/opt.h>
 #include <ccan/read_write_all/read_write_all.h>
 #include <ccan/str/hex/hex.h>
@@ -19,6 +18,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
+#include <lightningd/json.h>
 #include <lightningd/jsonrpc.h>
 #include <lightningd/lightningd.h>
 #include <lightningd/options.h>
@@ -31,39 +31,6 @@
 /* Once we're up and running, this is set up. */
 struct log *crashlog;
 
-struct log_entry {
-	struct list_node list;
-	struct timeabs time;
-	enum log_level level;
-	unsigned int skipped;
-	const char *prefix;
-	char *log;
-	/* Iff LOG_IO */
-	const u8 *io;
-};
-
-struct log_book {
-	size_t mem_used;
-	size_t max_mem;
-	void (*print)(const char *prefix,
-		      enum log_level level,
-		      bool continued,
-		      const struct timeabs *time,
-		      const char *str,
-		      const u8 *io, size_t io_len,
-		      void *arg);
-	void *print_arg;
-	enum log_level print_level;
-	struct timeabs init_time;
-
-	struct list_head log;
-};
-
-struct log {
-	struct log_book *lr;
-	const char *prefix;
-};
-
 static void log_to_file(const char *prefix,
 			enum log_level level,
 			bool continued,
@@ -664,17 +631,6 @@ static void add_skipped(struct log_info *info)
 	}
 }
 
-static 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);
-}
-
 static void log_to_json(unsigned int skipped,
 			struct timerel diff,
 			enum log_level level,