Decouple CChain from mapBlockIndex
Move CBlockIndex, CChain and related code out of main
CMessageHeader sanity changes

- Remove spurious `pchCommand[1] = 1` in CMessageHeader()
- Make sure that pchCommand is zero-padded if length is shorter than
  COMMAND_SIZE
- Use strnlen to determine length of pcmCommand in GetCommand
Write fee estimate and peers files only when initialized

Fixes #4669.

Move the loading of addresses to StartNode() to make it more
self-contained.
build: fix release name strings for gitian builds

When building from a distdir as gitian does, checking for the .git dir
is not reliable. Instead, ask git if we're in a repo.
Reserve only one network specific cached path per session
Remove CBaseChainParams::NetworkID()
remove code below asserts in limitedmap.h (fixes a ToDo)
[Qt] minor changes in splashscreen.cpp

- guard an unused variable with Q_UNUSED() macro
- remove a commented out line of code
Fixing out of bounds asses error
Avoiding referencing elements of an empty vector
Use ModifyCoins instead of mutable GetCoins.

Replace the mutable non-copying GetCoins method with a ModifyCoins, which
returns an encapsulated iterator, so we can keep track of concurrent
modifications (as iterators can be invalidated by those) and run cleanup
code after a modification is finished.

This also removes the overloading of the 'GetCoins' name.
Get rid of CCoinsView's SetCoins and SetBestBlock.

All direct modifications are now done through ModifyCoins, and BatchWrite is
used for pushing batches of queued modifications up, so we don't need the
low-level SetCoins and SetBestBlock anymore in the top-level CCoinsView class.
Do not keep fully spent but unwritten CCoins entries cached.

Instead of storing CCoins entries directly in CCoinsMap, store a CCoinsCacheEntry
which additionally keeps track of whether a particular entry is:
* dirty: potentially different from its parent view.
* fresh: the parent view is known to not have a non-pruned version.

This allows us to skip non-dirty cache entries when pushing batches of changes up,
and to remove CCoins entries about transactions that are fully spent before the
parent cache learns about them.
Add coins_tests with a large randomized CCoinViewCache test.
Reinitializing list's begin iterator after few elements were erased from the head
Get rid of the dummy CCoinsViewCache constructor arg
Do merkle root and txid duplicates check simultaneously

Move the txid duplicates check into BuildMerkleTree, where it can be done
much more efficiently (without needing to build a full txid set to detect
duplicates).

The previous version (using the std::set<uint256> to detect duplicates) was
also slightly too weak. A block mined with actual duplicate transactions
(which is invalid, due to the inputs of the duplicated transactions being
seen as double spends) would trigger the duplicates logic, resulting in the
block not being stored on disk, and rerequested. This change fixes that by
only triggering in the case of duplicated transactions that can actually
result in an identical merkle root.
Apply clang-format on crypto/* and compat/*
[Qt] add more NULL pointer checks in bitcoingui

- add missing checks for clientModel and optionsModel
- small cleanups for an #ifdef on Mac
- remove an unneeded else