Implements the "Cold Staking" system described in https://github.com/random-zebra/PIVX-Wiki/blob/master/Developer-Documentation/Specs/ColdStaking.mediawiki.

Closes #579.

The current changeset does not include any edit to the GUI, as it is being redesigned for v4.0.0 (#954).
The controls for cold staking will be added to the new interface in a separate PR if this upgrade is approved.

Enforcement activation height uses placeholder values for now.

Notable changes:

• [Core]

  • Definition of the new type of address STAKING_ADDRESS.
  • Definition of a new standard transaction type TX_COLDSTAKE.
  • Block signature/validation change: sign blocks with the staker private key, verify the signature extracting the public key from the coinstake input.
  • Addition of CheckColdStake function, called by CheckTransaction, to check for the rules not verified by the script evaluation (equivalence of input and output scripts).
  • Addition of -coldstaking startup flag to disable cold staking while keeping hot staking enabled (if -staking=1). If -staking=0 then -coldstaking is automatically set to 0 too.
  • Definition of placeholders for the activation block height: 2880000 for main net, 2106100for test net, 251 for reg test.
  • Definition of threshold value for cold staking outputs

• [Script]

  • Definition of a new opcode OP_CHECKCOLDSTAKEVERIFY
  • Definition of the output script P2CS
  • Definition of a fast test to recognize P2CS scripts by checking only few bytes.
  • Definition of the input scriptSig to spend P2CS scriptPubKey outputs
  • Addition of a boolean argument fColdStake to ExtractDestination to extract proper keyID from P2CS scripts.

• [Wallet]

  • Introduction of new ismine types to recognize cold staking txes: ISMINE_COLD, ISMINE_SPENDABLE_DELEGATED.
  • Introduction of the concepts of "delegated" and "cold" balance (debit and credit)
  • Addition of a new boolean flag fStakeDelegationVoided to walletTx when voiding stake delegations.
  • Addition of 2 new boolean optional arguments to AvailableCoins to include cold and/or delegated coins in the selection.
  • Inclusion of delegated coins in SelectStakeCoins for cold staking.
  • Addition of the delegator whitelisting check in AvailableCoins.

• [RPC]

  • Addition of an option to include delegated balance in getbalance, senfrom, sendmany and listtransactions.
  • Definition of new method getnewstakingaddress to generate a new staking address.
  • Definition of new method delegatestake to send a stake delegation transaction
  • Definition of new method delegatestakeraw to create a raw stake delegation transaction without sending it
  • Prevent the use of staking addresses as recipients in sendtoaddress.
  • Definition of new method getcoldstakingbalance to retrieve the cold balance of the wallet.
  • Addition of cold_staking_balance and delegated_balance to getwalletinfo/getinfo.
  • signrawtransaction: for P2CS, select the key used to sign based on what is found in the keystore. If the wallet has both keys, sign with the owner key.
  • Definition of new method delegatoradd to whitelist an owner address for cold staking.
  • Definition of new method delegatorremove to un-do the effect of delegatoradd.
  • Definition of new function listcoldutxos to show p2cs utxos belonging to the wallet (either as owner or staker).
  • Definition of new function liststakingaddresses to show the staking addresses generated by the wallet.
  • Definition of new function liststdelegators to show the delegator (owner) addresses whitelisted by the (cold) wallet.

• [Tests]

  • Creation of the new feature_coldStaking.py functional test and addition of it to the test-runner.

• [Misc/Clenup]

  • Remove extra declaration of EnsureWalletIsUnlocked in rpcdump.cpp
  • REGTEST: lower the confirmations to 15 (same as testnet) and fix the stakeModifier.