Redesign Ndb.swift interface with build safety

This commit redesigns the Ndb.swift interface with a focus on build-time
safety against crashes.

It removes the external usage of NdbTxn and SafeNdbTxn, restricting it
to be used only in NostrDB internal code.

This prevents dangerous and crash prone usages throughout the app, such
as holding transactions in a variable in an async function (which can
cause thread-based reference counting to incorrectly deinit inherited
transactions in use by separate callers), as well as holding unsafe
unowned values longer than the lifetime of their corresponding
transactions.

Closes: https://github.com/damus-io/damus/issues/3364
Changelog-Fixed: Fixed several crashes throughout the app
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

nostrdb/Ndb+.swift

@@ -29,8 +29,8 @@ extension Ndb {
     }
     
     /// Determines if a given note was seen on any of the listed relay URLs
-    func was(noteKey: NoteKey, seenOnAnyOf relayUrls: [RelayURL], txn: SafeNdbTxn<()>? = nil) throws -> Bool {
-        return try self.was(noteKey: noteKey, seenOnAnyOf: relayUrls.map({ $0.absoluteString }), txn: txn)
+    func was(noteKey: NoteKey, seenOnAnyOf relayUrls: [RelayURL]) throws -> Bool {
+        return try self.was(noteKey: noteKey, seenOnAnyOf: relayUrls.map({ $0.absoluteString }))
     }
     
     func processEvent(_ str: String, originRelayURL: RelayURL? = nil) -> Bool {