txindex: skip bloom filters and legacy lookups for new databases

New databases never contain legacy ('t' + txid) entries.
Peek at the database before opening it, and if no legacy
entries are found, skip building bloom filters (hashed
entries are only read via iterators, which do not consult
them) and return early from lookups instead of checking
for legacy entries.

src/dbwrapper.cpp

@@ -153,6 +153,30 @@ static leveldb::Options GetOptions(size_t nCacheSize, bool bloom_filter)
     return options;
 }
 
+bool CDBWrapper::HasKeyStartingWith(const fs::path& path, uint8_t prefix)
+{
+    if (!fs::exists(path / "CURRENT")) return false;
+
+    CBitcoinLevelDBLogger logger;
+    leveldb::Options options;
+    options.paranoid_checks = true;
+    // Avoid creating or rotating LevelDB's LOG files during this probe.
+    options.info_log = &logger;
+
+    leveldb::DB* raw_db;
+    HandleError(leveldb::DB::Open(options, fs::PathToString(path), &raw_db));
+    const std::unique_ptr<leveldb::DB> db{raw_db};
+
+    leveldb::ReadOptions iteroptions;
+    iteroptions.verify_checksums = true;
+    iteroptions.fill_cache = false;
+    const std::unique_ptr<leveldb::Iterator> it{db->NewIterator(iteroptions)};
+    const leveldb::Slice prefix_slice{reinterpret_cast<const char*>(&prefix), sizeof(prefix)};
+    it->Seek(prefix_slice);
+    HandleError(it->status());
+    return it->Valid() && it->key().starts_with(prefix_slice);
+}
+
 struct CDBBatch::WriteBatchImpl {
     leveldb::WriteBatch batch;
 };