refactor: convert tr_incomplete_metadata to c++ class (#6383)

* refactor: unset peer BEP-9 support if size hint is invalid

* fix: open torrent file in binary mode

* refactor: move metadata size check to method

* refactor: remove duplicate checks

* refactor: reduce temp variable scope in `parseLtepHandshake()`

* refactor: convert `get_piece_length()` to method

* refactor: convert `tr_torrentSetMetadataSizeHint()` to method

* refactor: convert `tr_torrentGetMetadataPiece()` to method

* refactor: convert `tr_torrentUseMetainfoFromFile()` to method

* refactor: convert `tr_torrentSetMetadataPiece()` to method

* refactor: convert `tr_torrentGetNextMetadataRequest()` to method

* refactor: convert `tr_torrentGetMetadataPercent()` to method

* refactor: add basic framework for MagnetMediator

* refactor: initialise `tr_incomplete_metadata` fields in constructor

* refactor: check metadata transfer completion in `set_metadata_piece()`

* refactor: convert `use_new_metainfo()` and `on_have_all_metainfo()` to methods

* refactor: move parts of `tr_torrent::set_metadata_piece()` into `tr_incomplete_metadata`

* refactor: move parts of `tr_torrent::get_next_metadata_request()` into `tr_incomplete_metadata`

* refactor: move parts of `tr_torrent::get_metadata_percent()` into `tr_incomplete_metadata`

* refactor: hide all `tr_incomplete_metadata` fields

* refactor: move `incomplete_metadata` to private

* feat: add test for `set_metadata_piece()`

* refactor: unify integer types

* refactor: rename `tr_incomplete_metadata` to `tr_metadata_download`

* chore: make clang-tidy happy

libtransmission/torrent-magnet.cc:117:68: warning: comparison of integers of different signs: 'long' and 'const uint64_t' (aka 'const unsigned long') [clang-diagnostic-sign-compare]

* refactor: pass log name to `tr_metadata_download` constructor

* chore: iwyu

* fix: thread-safe `TorrentMagnetTest.setMetadataPiece`

* chore: housekeeping

* Revert "fix: thread-safe `TorrentMagnetTest.setMetadataPiece`"

This reverts commit 2a7fcd93a262888f9f55d542b1a9a2da9ca72cea.

* fix: stop soon instead of stop now in `on_metainfo_completed()`

This is unreachable code now, but if it is ever reached, Transmission
will very likely crash.

* fix: maybe fix OpenBSD test failure

libtransmission/peer-mgr.cc

@@ -2256,7 +2256,6 @@ void tr_peerMgr::bandwidth_pulse()
     for (auto* const tor : torrents_)
     {
         tor->do_idle_work();
-        tr_torrentMagnetDoIdleWork(tor);
     }
 
     reconnect_pulse();