txn: do not close txn if database is already closed

This is a potential fix for some of the crash reports that have been
streaming in. They all seem to be crashing within ndb_close_txn. I
suspect this means that the transactions are trying to get closed when
ndb is already closed. Closing Ndb will close the transactions
automatically, so it looks like we might be trying to close twice.

damus/Models/Mute/MuteManager.swift

@@ -0,0 +1,8 @@
+//
+//  MuteManager.swift
+//  damus
+//
+//  Created by William Casarin on 2024-01-25.
+//
+
+import Foundation

nostrdb/NdbTxn.swift

@@ -17,9 +17,11 @@ class NdbTxn<T> {
     private var val: T!
     var moved: Bool
     var inherited: Bool
+    var ndb: Ndb
 
     init?(ndb: Ndb, with: (NdbTxn<T>) -> T = { _ in () }) {
         guard !ndb.closed else { return nil }
+        self.ndb = ndb
 #if TXNDEBUG
         txn_count += 1
         print("opening transaction \(txn_count)")
@@ -41,11 +43,12 @@ class NdbTxn<T> {
         self.val = with(self)
     }
 
-    private init(txn: ndb_txn, val: T) {
+    private init(ndb: Ndb, txn: ndb_txn, val: T) {
         self.txn = txn
         self.val = val
         self.moved = false
         self.inherited = false
+        self.ndb = ndb
     }
 
     /// Only access temporarily! Do not store database references for longterm use. If it's a primitive type you
@@ -56,7 +59,7 @@ class NdbTxn<T> {
     }
 
     deinit {
-        if moved || inherited {
+        if moved || inherited || ndb.closed {
             return
         }
 
@@ -71,14 +74,14 @@ class NdbTxn<T> {
     // functor
     func map<Y>(_ transform: (T) -> Y) -> NdbTxn<Y> {
         self.moved = true
-        return .init(txn: self.txn, val: transform(val))
+        return .init(ndb: self.ndb, txn: self.txn, val: transform(val))
     }
 
     // comonad!?
     // useful for moving ownership of a transaction to another value
     func extend<Y>(_ with: (NdbTxn<T>) -> Y) -> NdbTxn<Y> {
         self.moved = true
-        return .init(txn: self.txn, val: with(self))
+        return .init(ndb: self.ndb, txn: self.txn, val: with(self))
     }
 }
 
@@ -101,7 +104,7 @@ extension NdbTxn where T: OptionalType {
             return nil
         }
         self.moved = true
-        return NdbTxn<T.Wrapped>(txn: self.txn, val: unwrappedVal)
+        return NdbTxn<T.Wrapped>(ndb: self.ndb, txn: self.txn, val: unwrappedVal)
     }
 }