Fix app swap crash

This commit fixes a crash that occurred when swapping between Damus and
other apps.

When Damus enters background mode, NostrDB is closed and its resources
released. When Damus re-enters foreground mode, NostrDB is reopened.

However, an issue with the transaction inheritance logic
caused a race condition where a side menu profile lookup would get an
obsolete transaction containing pointers that have been freedwhen
NostrDB was closed, causing a "use-after-free" memory error.

The issue was fixed by improving the transaction inheritance logic to
double-check if the "generation" counter (which auto increments when
Damus closes and re-opens) matches the generation marked on the
thread-specific transaction. This effectively prevents lookups from
inheriting an obsolete transaction from a previous NostrDB generation.

Closes: https://github.com/damus-io/damus/issues/3167
Changelog-Fixed: Fixed an issue where the app would crash when swapping between apps
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

nostrdb/NdbTxn.swift

@@ -30,7 +30,10 @@ class NdbTxn<T>: RawNdbTxnAccessible {
         self.name = name ?? "txn"
         self.ndb = ndb
         self.generation = ndb.generation
-        if let active_txn = Thread.current.threadDictionary["ndb_txn"] as? ndb_txn {
+        if let active_txn = Thread.current.threadDictionary["ndb_txn"] as? ndb_txn,
+           let txn_generation = Thread.current.threadDictionary["txn_generation"] as? Int,
+           txn_generation == ndb.generation
+        {
             // some parent thread is active, use that instead
             print("txn: inherited txn")
             self.txn = active_txn
@@ -147,7 +150,10 @@ class SafeNdbTxn<T: ~Copyable> {
         var generation = ndb.generation
         var txn: ndb_txn
         let inherited: Bool
-        if let active_txn = Thread.current.threadDictionary["ndb_txn"] as? ndb_txn {
+        if let active_txn = Thread.current.threadDictionary["ndb_txn"] as? ndb_txn,
+           let txn_generation = Thread.current.threadDictionary["txn_generation"] as? Int,
+           txn_generation == ndb.generation
+        {
             // some parent thread is active, use that instead
             print("txn: inherited txn")
             txn = active_txn