Compilation error (std::placeholders) on Ubuntu 20+

[bitmarkcc]

/usr/include/boost/bind/bind.hpp:463:35: error: no match for call to ‘(boost::_mfi::mf3<void, CWalletInterface, const uint256&, const CTransaction&, const CBlock*>) (CWalletInterface*&, std::_Placeholder<1>&, std::_Placeholder<2>&, std::_Placeholder<3>&)’
/usr/include/boost/bind/bind.hpp:319:35: error: no match for call to ‘(boost::_mfi::mf1<void, CWalletInterface, const uint256&>) (CWalletInterface*&, std::_Placeholder<1>&)’
/usr/include/boost/bind/bind.hpp:319:35: error: no match for call to ‘(boost::_mfi::mf1<void, CWalletInterface, const CBlockLocator&>) (CWalletInterface*&, std::_Placeholder<1>&)’
/usr/include/boost/bind/bind.hpp:129:19: error: no match for ‘operator==’ (operand types are ‘const std::_Placeholder<1>’ and ‘const std::_Placeholder<1>’)
/usr/include/boost/bind/bind.hpp:129:19: error: no match for ‘operator==’ (operand types are ‘const std::_Placeholder<2>’ and ‘const std::_Placeholder<2>’)
/usr/include/boost/bind/bind.hpp:129:19: error: no match for ‘operator==’ (operand types are ‘const std::_Placeholder<3>’ and ‘const std::_Placeholder<3>’)

Solution: bitmarkcc@a8d080e

Also this commit retains consistency with boost::placeholders: bitmarkcc@ac2ec68