use @mmalmi json string preparsing speedup for duplicate events

- get rid of "nonUnique" variants of subMany as we can now relay on CheckDuplicate to track relays.
- add global pool tracker duplicateMiddleware.

subscription.go

@@ -31,6 +31,10 @@ type Subscription struct {
 	// Context will be .Done() when the subscription ends
 	Context context.Context
 
+	// if it is not nil, CheckDuplicate will be called for every event received
+	// if it returns true that event will not be processed further.
+	CheckDuplicate func(id string, relay string) bool
+
 	match  func(*Event) bool // this will be either Filters.Match or Filters.MatchIgnoringTimestampConstraints
 	live   atomic.Bool
 	eosed  atomic.Bool