Add BlockDX support

[smogm]

As Blocknet is a real DEX without any blockable custodian services like the gateways of bitshares, we should try to get a foot into the "real DEX" world.

The Blocknet BlockDX exchange has a short list of requirements a wallet daemon has to implement to be supported by the exchange:

https://docs.blocknet.co/blockdx/listings/

Especially the OP_CLTV has to be implemented
https://blog.decred.org/2017/09/20/On-Chain-Atomic-Swaps/

[HenryYoung42]

A quick check of eccoind command line help and source code shows that all the RPCs and OP_CLTV seem to be supported.