chore: reduce log buffer from 100MB to 10MB

The 100MB log buffer has been the biggest memory footprint for the daemon.
Keeping 10MB for emergency log dumps seems sufficient.
This has been mentioned in the last developer meeting.

Changelog-Changed: In-memory log buffer reduced from 100MB to 10MB

lightningd/lightningd.c

@@ -180,7 +180,7 @@ static struct lightningd *new_lightningd(const tal_t *ctx)
 	 * book to hold all the entries (and trims as necessary), and multiple
 	 * log objects which each can write into it, each with a unique
 	 * prefix. */
-	ld->log_book = new_log_book(ld, 100*1024*1024);
+	ld->log_book = new_log_book(ld, 10*1024*1024);
 	/*~ Note the tal context arg (by convention, the first argument to any
 	 * allocation function): ld->log will be implicitly freed when ld
 	 * is. */