… sporks

a5a20e8 [Tests] Add check for tip != pblock in zerocoin_rejection_tests (random-zebra)
f7a93cd [Refactoring] Cleanup and simplify ParseAndValidateZerocoinSpends (random-zebra)
b943f2c [Refactoring] Remove zerocoin disabling check in ATMP and CheckBlock (random-zebra)
f91e4df [Refactoring] Skip expensive ContainsZerocoins/HasZerocoinSpendInputs (random-zebra)
dd784bb [Cleanup] Deprecate SPORK 16 (zerocoin maintenance) (random-zebra)
179662c [Cleanup] Deprecate SPORK 18 (random-zebra)
f91161f [Validation] Reject under-minting blocks (random-zebra)
6c5884c [BUG] Remove double counted nValueIn for zerocoin txes (random-zebra)
78a5da8 [BUG] Remove spork-based CheckPublicCoinSpendVersion (random-zebra)
10db9bf [BUG] Fix ParseAndValidateZerocoinSpend(s) (random-zebra)

Pull request description:

  #2425 introduced two bugs which are preventing a successful chain sync from scratch on `master`:

  1. It abstracted a piece of code in `ParseAndValidateZerocoinSpend`, which is only parsing/validating a single zerocoin spend per tx, while a transaction could contain multiple spends. Fix it by returning an optional list of `CoinSpendValue`, instead of a singleton.

  2. It re-introduced in the validation the (unused-at-the-time) function `CheckPublicCoinSpendVersion`, which is now failing at the first block with (version 3) public spends.
  As all spork-based checks, this was supposed to be skipped when `fInitialBlockDownload`.
  Zerocoin transactions are no longer accepted (in both nets) in the mempool, as well as inside blocks (after v5 enforcement), so we can just remove this obsolete function.

  The third commit fixes a very old bug: a zerocoin spend tx `nValueIn` is counted twice in ConnectBlock (first directly, after the coinspend parsing/validation, and then via `CCoinsViewCache::GetValueIn`).

  Then, since we are at it, we can deprecate `SPORK_16_ZEROCOIN_MAINTENANCE_MODE` and `SPORK_18_ZEROCOIN_PUBLICSPEND_V4`, which are now unused/unneeded.

  The last three commits are performance optimizations to avoid multiple calls to `ContainsZerocoins()`/`HasZerocoinSpendInputs()` in `ConnectBlock`, when possible.

ACKs for top commit:
  Fuzzbawls:
    ACK a5a20e8
  furszy:
    ACK a5a20e8

Tree-SHA512: 961a755fd36cb24a83d9f0382253cb6f6a94d06534104a3d3328db9047ff769a9ae83c27a4d538ce38c1d9dc25bd4da21ee4387c8ea6251b03d83f81270cd4cc