Vulnerability: CryptoKitty and CryptoFighter NFT can be paused, which block borrowing / repaying / liquidating action in the ERC721Pool when borrowers still forced to pay the compounding interest
Severity: medium

Description:
CryptoKitty and CryptoFighter NFT can be paused, which block borrowing / repaying / liquidating action in the ERC721Pool\\nIn the current implementation in the factory contract and the pool contract, special logic is in-place to handle non-standard NFT such as crypto-kitty, crypto-figher or crypto punk.\\nIn the factory contract:\\n```\\nNFTTypes nftType;\\n// CryptoPunks NFTs\\nif (collateral_ == 0xb47e3cd837dDF8e4c57F05d70Ab865de6e193BBB ) {\\n nftType = NFTTypes.CRYPTOPUNKS;\\n}\\n// CryptoKitties and CryptoFighters NFTs\\nelse if (collateral_ == 0x06012c8cf97BEaD5deAe237070F9587f8E7A266d || collateral_ ==  0x87d598064c736dd0C712D329aFCFAA0Ccc1921A1) {\\n nftType = NFTTypes.CRYPTOKITTIES;\\n}\\n// All other NFTs that support the EIP721 standard\\nelse {\\n // Here 0x80ac58cd is the ERC721 interface Id\\n // Neither a standard NFT nor a non-standard supported NFT(punk, kitty or fighter)\\n try IERC165(collateral_).supportsInterface(0x80ac58cd) returns (bool supportsERC721Interface) {\\n  if (!supportsERC721Interface) revert NFTNotSupported();\\n } catch {\\n  revert NFTNotSupported();\\n }\\n\\n nftType = NFTTypes.STANDARD_ERC721;\\n}\\n```\\n\\nAnd in ERC721Pool When handling ERC721 token transfer:\\n```\\n/**\\n *  @notice Helper function for transferring multiple NFT tokens from msg.sender to pool.\\n *  @notice Reverts in case token id is not supported by subset pool.\\n *  @param  poolTokens_ Array in pool that tracks NFT ids (could be tracking NFTs pledged by borrower or NFTs added by a lender in a specific bucket).\\n *  @param  tokenIds_   Array of NFT token ids to transfer from msg.sender to pool.\\n */\\nfunction _transferFromSenderToPool(\\n uint256[] storage poolTokens_,\\n uint256[] calldata tokenIds_\\n) internal {\\n bool subset   = _getArgUint256(SUBSET) != 0;\\n uint8 nftType = _getArgUint8(NFT_TYPE);\\n\\n for (uint256 i = 0; i < tokenIds_.length;) {\\n  uint256 tokenId = tokenIds_[i];\\n  if (subset && !tokenIdsAllowed[tokenId]) revert OnlySubset();\\n  poolTokens_.push(tokenId);\\n\\n  if (nftType == uint8(NFTTypes.STANDARD_ERC721)){\\n   _transferNFT(msg.sender, address(this), tokenId);\\n  }\\n  else if (nftType == uint8(NFTTypes.CRYPTOKITTIES)) {\\n   ICryptoKitties(_getArgAddress(COLLATERAL_ADDRESS)).transferFrom(msg.sender ,address(this), tokenId);\\n  }\\n  else{\\n   ICryptoPunks(_getArgAddress(COLLATERAL_ADDRESS)).buyPunk(tokenId);\\n  }\\n\\n  unchecked { ++i; }\\n }\\n}\\n```\\n\\nand\\n```\\nuint8 nftType = _getArgUint8(NFT_TYPE);\\n\\nfor (uint256 i = 0; i < amountToRemove_;) {\\n uint256 tokenId = poolTokens_[--noOfNFTsInPool]; // start with transferring the last token added in bucket\\n poolTokens_.pop();\\n\\n if (nftType == uint8(NFTTypes.STANDARD_ERC721)){\\n  _transferNFT(address(this), toAddress_, tokenId);\\n }\\n else if (nftType == uint8(NFTTypes.CRYPTOKITTIES)) {\\n  ICryptoKitties(_getArgAddress(COLLATERAL_ADDRESS)).transfer(toAddress_, tokenId);\\n }\\n else {\\n  ICryptoPunks(_getArgAddress(COLLATERAL_ADDRESS)).transferPunk(toAddress_, tokenId);\\n }\\n\\n tokensTransferred[i] = tokenId;\\n\\n unchecked { ++i; }\\n}\\n```\\n\\nnote if the NFT address is classified as either crypto kitties or crypto fighers, then the NFT type is classified as CryptoKitties, then transfer and transferFrom method is triggered.\\n```\\nif (nftType == uint8(NFTTypes.CRYPTOKITTIES)) {\\n   ICryptoKitties(_getArgAddress(COLLATERAL_ADDRESS)).transferFrom(msg.sender ,address(this), tokenId);\\n  }\\n```\\n\\nand\\n```\\nelse if (nftType == uint8(NFTTypes.CRYPTOKITTIES)) {\\n ICryptoKitties(_getArgAddress(COLLATERAL_ADDRESS)).transfer(toAddress_, tokenId);\\n}\\n```\\n\\nHowever, in both crypto-kitty and in crypto-figher NFT, the transfer and transferFrom method can be paused.\\nIn crypto-figher NFT:\\n```\\nfunction transferFrom(\\n address _from,\\n address _to,\\n uint256 _tokenId\\n)\\n public\\n whenNotPaused\\n{\\n```\\n\\nIn Crypto-kitty NFT:\\n```\\nfunction transferFrom(\\n address _from,\\n address _to,\\n uint256 _tokenId\\n)\\n external\\n whenNotPaused\\n{\\n```\\n\\nnote the WhenNotPaused modifier.

Impact:
If the transfer and transferFrom is paused in CryptoKitty and CryptoFighter NFT, the borrowing and repaying and liquidating action is blocked in ERC721Pool, the user cannot fully clear his debt and has to pay the compounding interest when the transfer is paused.

Recommended mitigation:
Issue CryptoKitty and CryptoFighter NFT can be paused, which block borrowing / repaying / liquidating action in the ERC721Pool when borrowers still forced to pay the compounding interest\\nInterest should not be charged when external contract is paused to borrower when the external contract pause the transfer and transferFrom.