Add tweak_fg_sleep feature set for ffnet, make ffnet chapter look ahead optional

plugin-defaults.ini

@@ -1207,6 +1207,15 @@ never_make_cover: true
 ## reducing this sleep.
 slow_down_sleep_time:4
 
+## ffnet is sensitive to too many hits.  Users are sensitive to long
+## waits during the initial metadata collection in the foreground.
+## When used, these settings will speed up metadata downloads in the
+## foreground linearly.
+tweak_fg_sleep:true
+min_fg_sleep:1.0
+max_fg_sleep:4.0
+max_fg_sleep_at_downloads: 10
+
 ## Extra metadata that this adapter knows about.  See [dramione.org]
 ## for examples of how to use them.
 extra_valid_entries:reviews,favs,follows
@@ -1215,13 +1224,20 @@ extra_valid_entries:reviews,favs,follows
 ## to be romantic pairings.
 ships_label:Pairings
 
-## Date formats used by FFDL.  Published and Update don't have time.
+## Date formats used by FFDL.  Published and Update don't usually have
+## time, but they do now on ffnet.
 ## See http://docs.python.org/library/datetime.html#strftime-strptime-behavior
 ## Note that ini format requires % to be escaped as %%.
 #dateCreated_format:%%Y-%%m-%%d %%H:%%M:%%S
 datePublished_format:%%Y-%%m-%%d %%H:%%M:%%S
 dateUpdated_format:%%Y-%%m-%%d %%H:%%M:%%S
 
+## ffnet used to have a tendency to send out update notices in email
+## before all their servers were showing the update on the first
+## chapter.  It generates another server request and doesn't seem to
+## be needed lately, so now default it to off.
+check_next_chapter:false
+
 [www.fanfiktion.de]
 ## Some sites require login (or login for some rated stories) The
 ## program can prompt you, or you can save it in config.  In