Build the python bindings (and the library) with the oldest ABI view, TORRENT_ABI_VERSION=1.

libtorrent 2.1 hides the API that deluge-2.2.0 still calls: those entries are guarded by
'#if TORRENT_ABI_VERSION == 1' inside the bindings themselves -
  * add_torrent_params['resume_data'] (bindings/python/src/session.cpp): without that key
    session.add_torrent() raises KeyError 'unknown name in torrent params: resume_data',
    which emptied the torrent library under 2.1.2 - the daemon re-adds its saved torrents
    with fast-resume data at startup.
  * torrent_status::paused / ::auto_managed (bindings/python/src/torrent_status.cpp): read
    by deluge/core/torrent.py in nine places (the 'no attribute paused' tracebacks).
  * enc_level aliases plaintext/rc4/both (bindings/python/src/session_settings.cpp): 2.1
    ships only pe_plaintext/pe_rc4/pe_both, and deluge/core/preferencesmanager.py uses the
    old spellings (the per-minute 'no attribute plaintext' tracebacks).

libtorrent's own default (its 'deprecated-functions' cmake feature) selects ABI 2;
include/libtorrent/aux_/export.hpp numbers the macros 1 = libtorrent-1.1 ... 4 =
libtorrent-2.1, so ABI 1 is the backwards-compatible build. It keeps the newer names the
bindings gate at 'TORRENT_ABI_VERSION < 4' (torrent_handle.torrent_file(),
get_renamed_files()), so it still pairs with the deluge#532 patch folded into
deluge-2.2.0-r4. No deluge-side patching for these four call sites is then needed.
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -739,7 +739,7 @@
 feature_option(dht "enable support for Mainline DHT" ON)
 target_optional_compile_definitions(torrent-rasterbar PUBLIC FEATURE NAME deprecated-functions DEFAULT ON
 	DESCRIPTION "enable deprecated functions for backwards compatibility"
-	ENABLED TORRENT_ABI_VERSION=2
+	ENABLED TORRENT_ABI_VERSION=1
 	DISABLED TORRENT_ABI_VERSION=100)
 feature_option(encryption "Enables encryption in libtorrent" ON)
 feature_option(exceptions "build with exception support" ON)