refactor: have CCoins* data managed under CChainState

This change encapsulates UTXO set data within CChainState instances, removing
global data `pcoinsTip` and `pcoinsviewdb`. This is necessary if we want to
maintain multiple chainstates with their own rendering of the UTXO set.

We introduce a class CoinsViews which consolidates the construction of a
CCoins* hierarchy. Construction of its various pieces (db, coinscatcher,
in-memory cache) is split up so that we avoid flushing bad state to disk if
startup is interrupted.

We also introduce `CChainState::CanFlushToDisk()` which tells us when it is
safe to flush the chainstate based on this partial construction.

This commit could be broken into smaller pieces, but it would require more
ephemeral diffs to, e.g., temporarily change CCoinsViewDB's constructor
invocations.

Other changes:

- A parameter has been added to the CCoinsViewDB constructor that allows the
  name of the corresponding leveldb directory to be specified.

Thanks to Russell Yanofsky and Marco Falke for helpful feedback.

src/txdb.cpp

@@ -52,7 +52,7 @@ struct CoinEntry {
 
 }
 
-CCoinsViewDB::CCoinsViewDB(size_t nCacheSize, bool fMemory, bool fWipe) : db(GetDataDir() / "chainstate", nCacheSize, fMemory, fWipe, true)
+CCoinsViewDB::CCoinsViewDB(fs::path ldb_path, size_t nCacheSize, bool fMemory, bool fWipe) : db(ldb_path, nCacheSize, fMemory, fWipe, true)
 {
 }