Merge #15976: refactor: move methods under CChainState (pt. 1)

403e677c9 refactoring: IsInitialBlockDownload -> CChainState (James O'Beirne)
3ccbc376d refactoring: FlushStateToDisk -> CChainState (James O'Beirne)
4d6688603 refactoring: introduce ChainstateActive() (James O'Beirne)
d7c97edee move-only: make the CChainState interface public (James O'Beirne)

Pull request description:

  This is part of the [assumeutxo project](https://github.com/bitcoin/bitcoin/projects/11):

  Parent PR: #15606
  Issue: #15605
  Specification: https://github.com/jamesob/assumeutxo-docs/tree/2019-04-proposal/proposal

  ---

  This changeset starts moving functionality intimately related to CChainState into methods. Parameterizing these functions by a particular CChainState is necessary for the use of multiple chainstates simultaneously (e.g. for asynchronous background validation).

  In this change, we
  - make the CChainState interface public - since other units will start to invoke its methods directly,
  - introduce `::ChainstateActive()`, the CChainState equivalent for `::ChainActive()`,
  - and move `IsInitialBlockDownload()` and `FlushStateToDisk()` into methods on CChainState.

  Independent of assumeutxo, these changes better encapsulate chainstate behavior and allow easier use from a testing context.

  There are more methods that we'll move in the future, but they require other substantial changes (i.e. moving ownership of the `CCoinsView*` hierarchy into CChainState) so we'll save them for future PRs.

  ---

  The first move-only commit is most easily reviewed with `git diff ... --color-moved=dimmed_zebra`.

ACKs for commit 403e67:
  Empact:
    utACK 403e677c9e no need to address my nits herein
  Sjors:
    utACK 403e677
  ryanofsky:
    utACK 403e677c9e. Only change since previous review is removing global state comment as suggested.
  MarcoFalke:
    utACK 403e677c9e, though the diff still seems a bit bloated with some unnecessary changes in the second commit.
  promag:
    utACK 403e677 and rebased with current [master](c7cfd20a7).

Tree-SHA512: 6fcf260bb2dc201361170c0b4547405366f5f331fcc3a2bac29b24442814b7b244ca1b58aac5af716885f9a130c343b544590dff780da0bf835c7c5b3ccb2257

src/interfaces/chain.cpp

@@ -334,8 +334,8 @@ public:
         return ::fHavePruned;
     }
     bool p2pEnabled() override { return g_connman != nullptr; }
-    bool isReadyToBroadcast() override { return !::fImporting && !::fReindex && !IsInitialBlockDownload(); }
-    bool isInitialBlockDownload() override { return IsInitialBlockDownload(); }
+    bool isReadyToBroadcast() override { return !::fImporting && !::fReindex && !isInitialBlockDownload(); }
+    bool isInitialBlockDownload() override { return ::ChainstateActive().IsInitialBlockDownload(); }
     bool shutdownRequested() override { return ShutdownRequested(); }
     int64_t getAdjustedTime() override { return GetAdjustedTime(); }
     void initMessage(const std::string& message) override { ::uiInterface.InitMessage(message); }

src/interfaces/node.cpp

@@ -197,7 +197,7 @@ public:
         }
         return GuessVerificationProgress(Params().TxData(), tip);
     }
-    bool isInitialBlockDownload() override { return IsInitialBlockDownload(); }
+    bool isInitialBlockDownload() override { return ::ChainstateActive().IsInitialBlockDownload(); }
     bool getReindex() override { return ::fReindex; }
     bool getImporting() override { return ::fImporting; }
     void setNetworkActive(bool active) override