With rustls it's easier to embed the root CA certificates inside
the compiled binary itself using the webpki-roots crate. We need to
do this because it's the easiest way of getting TLS certificate
validation working inside the initramfs where /etc/ssl/certs or
else does not exist.

In contradiction with Jean-Paul's guidelines on not using Rust due
to lack of knowledge about it inside Nexedi, I am using it here
because it is the fastest way for me to get a working standalone
static binary, I know that language best. Considering we must be
getting results ASAP, this is the best strategy for me. We may
later rewrite it in another language if necessary.

A shell script is included to build the static binary, you need
to install rustup to get rust for musl, an alternative libc that
allows to create real static binaries that embed libc itself too.

Rustup can be found at: https://rustup.rs/

You can get a musl toolchain with:
  $ rustup target add x86_64-unknown-linux-musl

The acl library is being downloaded and built as a static library
by the script, and the rust build system will also build a vendored
copy of openssl as a static library.

Parallel hashing is done a bit differently in that Rust version,
only files contained in the currently processed directories will be
hashed in parallel. If there is a single big file in a directory
hashing will be stuck on that file until it's done and it goes onto
the next directory. To clarify, each file is only hashed on a
single thread, the Python version also does this, it just keeps the
number of files being hashed in parallel to a constant number as
long as there is more files to process, this version will only hash
with one thread per file in the currently processed directory. It
was done that way for sake of simplicity but we can implement an
offload threadpool to mimick what was done in Python later on.

TODO: Find a way to properly increment version without having to
      store any additional state client-side

TODO: Investigate using HATEOAS to talk to ERP5

TODO: Investigate using TLS client certificates to authenticate,
      they would be stored in /boot and would prevent the machine
      from booting if they were invalid or missing so that
      tampering with them is not interesting for an attacker.
      Also, the certificate's Common Name should be the computer
      reference and therefore should be used to construct the
      metadata snapshot document's reference instead of having
      to specify it on the command line.

* Convert stat_result to proper dictionary so that field names are
  retained after serialization

* Add ability to ignore directories through command line arguments,
  explicitly add "ignored" field on ignored directories

It was decided that JSON was not a suitable format because bytes
serialization support is lacking. MsgPack supports it and is more
efficient, also it is the internal serialization format for Fluentd
which we will most probably use for ingesting data in a central
place.

multiprocessing.Pool.close() ensures no new tasks can be submitted
to the pool and waits for them to all finish. Even though
AsyncResult.get() also waits for the tasks to finish, and our code
structure shouldnt submit new tasks at that point, close() first,
get() then. In the future this could be error-prone in the future
where mp_tasks is modified while results are being merged back and
we miss some results because the iterator wont take these new items
into account *during* iteration.

In Python, the JSON encoder cannot process bytes, the JSON
specification also does not define a "bytes" type. We are
constrained by this in that we cannot serialize data of bytes type.

xattrs can be either strings or bytes, in practice they're likely
representable as strings, therefore, decode as utf-8, error
otherwise. If real world situation of xattrs in true binary format
arise then we will rule out another solution.