..and at the same time prevent AcceptBlockHeader() from calling global function Params()

-ProcessNewBlock
-TestBlockValidity

-BitcoinMiner
-CreateNewBlock
-GenerateBitcoins
-ProcessBlockFound

1) Chainparams: Explicit CChainParams arg for main:

-AcceptBlock
-AcceptBlockHeader
-ActivateBestChain
-ConnectTip
-InitBlockIndex
-LoadExternalBlockFile
-VerifyDB parametric constructor

2) Also pickup more Params()\. in main.cpp

3) Pass nPruneAfterHeight explicitly to new FindFilesToPrune() in main.cpp

Previously didn't make clear that the ContextualCheckBlock* functions
meant the block headers as context - not the UTXO set itself - and that
ConnectBlock() also did UTXO-related validity checks (in the future we
may split that functionality into a separate UTXO-specific contextual
check block function).

Also, reordered to put validity checks first for better readability.

Break the circular dependency between main and txdb by:

- Moving `CBlockFileInfo` from `main.h` to `chain.h`. I think this makes
  sense, as the other block-file stuff is there too.

- Moving `CDiskTxPos` from `main.h` to `txdb.h`. This type seems
  specific to txdb.

- Pass a functor `insertBlockIndex` to `LoadBlockIndexGuts`. This leaves
  it up to the caller how to insert block indices.

Zcash: This does not actually break the circular dependency for us yet, as we
still need to pull in bitcoin/bitcoin#7756 and bitcoin/bitcoin#7904.

- CheckBlockHeader
- CheckBlock

…ction()

- ContextualCheckBlockHeader
- ContextualCheckBlock

…ions()

…r=str4d

Remove old mergetoaddress RPC test

The test wallet_mergetoaddress.py is no longer run and has been replaced by mergetoaddress_sprout.py and mergetoaddress_sapling.py. It should have been removed as part of zcash#3619, but was not.

Bitcoin Core refactoring and cleanups 1

Cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#6538
- bitcoin/bitcoin#6163
- bitcoin/bitcoin#6982
- bitcoin/bitcoin#6986
- bitcoin/bitcoin#7053
- bitcoin/bitcoin#7444
- bitcoin/bitcoin#7793
  - Excluding some comments in `txmempool.h` on code we haven't yet pulled in.
- bitcoin/bitcoin#7916
- bitcoin/bitcoin#7815

Additionally, the Equihash parameters are moved into the consensus parameters, and
various other functions in `main.cpp` have `const CChainParams&` arguments added.

Part of zcash#2074.

Replace CSproutNotePlaintextEntry with SproutNoteEntry to match Sapling

This refactoring makes the output of GetFilteredNotes safer to use, by setting the Sprout note at the point of decryption, where the corresponding address is already known, rather than having to recall it using the correct payment address.

Fix typo and clean up help message for RPC z_getmigrationstatus.

z_getmigrationstatus help message wording change

This is follow up from zcash#4017