Merge #15118: Refactor block file logic

04cca33094 Style cleanup. (Jim Posen)
4c01e4e159 flatfile: Unit tests for FlatFileSeq methods. (Jim Posen)
65a489e93d scripted-diff: Rename CBlockDiskPos to FlatFilePos. (Jim Posen)
d6d8a78f26 Move CDiskBlockPos from chain to flatfile. (Jim Posen)
e0380933e3 validation: Refactor file flush logic into FlatFileSeq. (Jim Posen)
992404b31e validation: Refactor block file pre-allocation into FlatFileSeq. (Jim Posen)
e2d2abb99f validation: Refactor OpenDiskFile into method on FlatFileSeq. (Jim Posen)
9183d6ef65 validation: Extract basic block file logic into FlatFileSeq class. (Jim Posen)
62e7addb63 util: Move CheckDiskSpace to util. (Jim Posen)

Pull request description:

  This cleans up and refactors block file helpers so that they may be used by the block filter indexer. Per [design discussion](https://github.com/bitcoin/bitcoin/pull/14121#issuecomment-451252591) about storing BIP 157 block filters, it has been suggested that they are stored in the same way as block and undo data. This refactor is sufficient to simplify file operations for this use case, though in the future perhaps more pruning-related logic ought to be moved into the new classes.

  The basic abstraction is a `FlatFileSeq` which manages access to a sequence of numbered files into which raw data is written.

Tree-SHA512: b2108756777f2dad8964a1a2ef2764486e708a4a4a8cfac47b5de8bcb0625388964438eb096b10cfd9ea39212c299b5cb32fa943e768db2333cf49ea7def157e

src/util/system.h

@@ -72,6 +72,7 @@ bool RenameOver(fs::path src, fs::path dest);
 bool LockDirectory(const fs::path& directory, const std::string lockfile_name, bool probe_only=false);
 void UnlockDirectory(const fs::path& directory, const std::string& lockfile_name);
 bool DirIsWritable(const fs::path& directory);
+bool CheckDiskSpace(const fs::path& dir, uint64_t additional_bytes = 0);
 
 /** Release all directory locks. This is used for unit testing only, at runtime
  * the global destructor will take care of the locks.