Bitcoin Unlimited - Bitcoin Cash release 1.6.0.0

This is a main release candidate of Bitcoin Unlimited compatible
with the Bitcoin Cash specifications you could find here:

• https://github.com/bitcoincashorg/bitcoincash.org/blob/master/spec/uahf-technical-spec.md (Aug 1st '17 Protocol Upgrade, bucash 1.1.0.0)
• https://github.com/bitcoincashorg/bitcoincash.org/blob/master/spec/nov-13-hardfork-spec.md (Nov 13th '17 Protocol Upgrade, bucash 1.1.2.0)
• https://github.com/bitcoincashorg/bitcoincash.org/blob/master/spec/may-2018-hardfork.md (May 15th '18 Protocol Upgrade, bucash 1.3.0.0, 1.3.0.1, 1.4.0.0)
• https://github.com/bitcoincashorg/bitcoincash.org/blob/master/spec/2018-nov-upgrade.md (Nov 15th '18 Protocol Upgrade, bucash 1.5.0.0, 1.5.0.1, 1.5.0.2, 1.5.1.0)
• https://github.com/bitcoincashorg/bitcoincash.org/blob/master/spec/2019-05-15-upgrade.md (May 15th '19 Protocol Upgrade, bucash 1.6.0.0)

This release is compatible with Bitcoin Cash upcoming May 15th, 2019 protocol upgrade and SV changes to the consensus rules. SV features set is disabled by default, the default policy is to activate the set of changes as defined by the bitcoincash.org.

Main Changes in 1.6.0

This release will be compatible with the upcoming May, 15th 2019 BCH protocol upgrade. It improves
Graphene block propagation technique greatly and also implelemt Compact Blocks (BIP 152). This is list of the main changes that have
been merged in this release:

• Segwit P2SH recovery
• Schnorr signatures
• Compact Block implementations
• Basic integration with the electrum server electrs (experimental, off by default), see this doc for more info
• Graphene: do not send txns oders by leveraging CTOR
• Graphene: Fast Filter instead of Bloom Filter (optional)
• Graphene: seeding of IBLT hash function
• Graphene: use cheap hashes to avoid short id tx collision
• Graphene: reduce decode failure to 0.5% via IBLT padding
• Graphene: O(1) heuristic to determine IBLT / BF size for block over 600 txns
• RPC enhancements and new commands (getrawblocktransactions, getrawtransactionssince and gettxoutproofs)
• QA reliability improvemennts
• Easier gitian build process (docker based)
• Code restructure around the 3 block propagation techs we currently support (Graphene, CB, Xthin)
• Move to C++14 and bump lib boost minimum ver to match
• Update Windows build scripts
• Simplify and update Xpedited code
• Refactor Script interpreter as a "virtual machine" encapsulated by a class. This allows the script interpreter to be used outside of bitcoind (for example, its ability to "step" allows use in a script debugger).
• Implement Xversion changeable key
• Documentation improvements