Extension uninstall mapping causes libtorrent assert failure in debug mode

[mnaamani]

When we stop the extension we send an uninstall mapping where all the joystream extension message IDs are set to 0. This is leading to the TORRENT_ASSERT failure in below code.

In bt_peer_connection.cpp - write_extensions()

#ifndef NDEBUG
		// make sure there are not conflicting extensions
		std::set<int> ext;
		for (entry::dictionary_type::const_iterator i = m.begin()
			, end(m.end()); i != end; ++i)
		{
			if (i->second.type() != entry::int_t) continue;
			int val = int(i->second.integer());
			TORRENT_ASSERT(ext.find(val) == ext.end()); // Failed Assert
			ext.insert(val);
		}
#endif

We might want to consider having a method for directly writing the uninstall mapping to the peer to bypass libtorrent's checking.

This isn't a serious issue when running in Release mode but it would be nice to fix so when we are testing in Debug mode we don't get the failed assert.

[mnaamani]

A better approach to send "uninstall mappings" is to just not include the joystream extended messages in the m dictionary of the handshake.

This appears to be the way libtorrent considers an extension to be unsupported (in addition to value being set to 0)
In void bt_peer_connection::on_extended_handshake()

// upload_only
if (bdecode_node m = root.dict_find_dict("m"))
{
   m_upload_only_id = boost::uint8_t(m.dict_find_int_value("upload_only", 0));
   m_holepunch_id = boost::uint8_t(m.dict_find_int_value("ut_holepunch", 0));
   m_dont_have_id = boost::uint8_t(m.dict_find_int_value("lt_donthave", 0));
}

Eventhough the extension protocol specs mention that the m dict in subsequent handshakes should be treated additively, libtorrent doesn't treat it that way.