ndb/txn: inherit active transactions on the same thread

Many different parts of the codebase could be opening transactions when
somewhere higher in the heirarchy on the main thread might already have
an active transaction. This can lead to failed transaction opening which
is bad.

Instead of relying on passing down the transaction to subviews, lets
keep track of the active transactions in a thread-local dictionary. That
way whenever we create a new transaction we can inherit the one that is
already active in the current thread.

Inherited transactions don't end the query when they are garbage
collected, we still expect the first-opened query to do this.

nostrdb/Test/NdbTests.swift

@@ -155,6 +155,25 @@ final class NdbTests: XCTestCase {
         let testNote = NdbNote.owned_from_json(json: testJSONWithEscapedSlashes)!
         XCTAssertEqual(testNote.content, "https://cdn.nostr.build/i/5c1d3296f66c2630131bf123106486aeaf051ed8466031c0e0532d70b33cddb2.jpg")
     }
+    
+    func test_inherited_transactions() throws {
+        let ndb = Ndb(path: db_dir)!
+        do {
+            let txn1 = NdbTxn(ndb: ndb)
+            
+            let ntxn = (Thread.current.threadDictionary.value(forKey: "ndb_txn") as? ndb_txn)!
+            XCTAssertEqual(txn1.txn.lmdb, ntxn.lmdb)
+            XCTAssertEqual(txn1.txn.mdb_txn, ntxn.mdb_txn)
+
+            let txn2 = NdbTxn(ndb: ndb)
+
+            XCTAssertEqual(txn1.inherited, false)
+            XCTAssertEqual(txn2.inherited, true)
+        }
+
+        let ndb_txn = Thread.current.threadDictionary.value(forKey: "ndb_txn")
+        XCTAssertNil(ndb_txn)
+    }
 
     func test_decode_perf() throws {
         // This is an example of a performance test case.