Draft: Add Decred Chain

Current status: Mocknet add liquidity and swap passes.

Test script on mocknet: snippet

Relies on the following libaries/tools:

forked dcrwallet https://github.com/decred/dcrwallet/tree/release-v1.6

forked TSS dcrd-txdcript based on v1.6.2 release: https://gitlab.com/vikingshield/dcrd-txscript

Decred docker image: https://gitlab.com/vikingshield/decred

Related to #991 (closed)