updated xenial and stretch dockerfiles since they bundle old versions of libsodium
refacatoring to make BlockchainInterface and BitcoinCoreNoHistoryInterface python2 compatible
add the -acceptnonstdtxn flag when invoking bitcoind for versions >= 0.19.0
refactor the use of nonlocal to make it compatible with python2
document the get_bitcoind_version method
Carl Dong
undeath
Tim Akinbo
fivepiece
James Hilliard