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/init.cpp

@@ -259,7 +259,7 @@ void Shutdown(InitInterfaces& interfaces)
 
     // FlushStateToDisk generates a ChainStateFlushed callback, which we should avoid missing
     if (pcoinsTip != nullptr) {
-        FlushStateToDisk();
+        ::ChainstateActive().ForceFlushStateToDisk();
     }
 
     // After there are no more peers/RPC left to give us new data which may generate
@@ -275,7 +275,7 @@ void Shutdown(InitInterfaces& interfaces)
     {
         LOCK(cs_main);
         if (pcoinsTip != nullptr) {
-            FlushStateToDisk();
+            ::ChainstateActive().ForceFlushStateToDisk();
         }
         pcoinsTip.reset();
         pcoinscatcher.reset();
@@ -1693,7 +1693,7 @@ bool AppInitMain(InitInterfaces& interfaces)
         nLocalServices = ServiceFlags(nLocalServices & ~NODE_NETWORK);
         if (!fReindex) {
             uiInterface.InitMessage(_("Pruning blockstore..."));
-            PruneAndFlush();
+            ::ChainstateActive().PruneAndFlush();
         }
     }