feat(utxo-swap): add utxo burn output for non-kmd taker fee

[dimxy]

Adds a burn output sending 25% of the taker utxo DEX fee to a dedicated burn address. Funds collected on the burn address will be traded for KMD to burn them (thus additionally burning KMD supply).
This PR partially closes #2010

In this PR:

• split dex fee for non-kmd utxo coins (apart from zcoin) and added an output to burn account
• old-style non-split dex fee non-kmd txns are also allowed in validation code (until all taker nodes upgraded)
• refactored dex fee pubkey in params: instead of passing dex fee and burn pubkeys in function params new methods dex_pubkey() and burn_pubkey() were added to the SwapOps trait
• mocktopus was made optional dependency and activated only for development builds (as its doc suggests).

NOTE: As mocktopus now is marked 'optional = true' in coins Cargo.toml and activated from the mm2_main crate by adding features = ["mocktopus"] in [dev-dependencies] section, you also need to mark your mockable code, called from other crates, this way: #[cfg_attr(feature = "mocktopus", mockable)], otherwise mocks won't work (see samples in code)

TODO:

• fix burn pubkey and burn zaddr
• Add private burn output for zcoin
• disable non-split non-kmd dex fee (no burn output) validation when all taker nodes upgrade to new dex fee splitting