Harden index reads

[SirTyson]

Description

Resolves #4546

This change calls fsync after writing BucketIndex files. This change was slightly more involved than expected. There's no way to fall fsync on files opened via std::fstream, so I had to implement a new cereal archive type that accepts our own asio::buffered_write_streamasio::buffered_write_stream backed stream type.

Rebased on #4403.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document