Minor bugfix release

* crypto API improvements
* python bindings improvements
* implement first python unit tests
* minor bug fixes

MSVC: decouple argon2 dependency

Refactor Identity and PrivateKey to use
shared_ptr<PrivateKey> and retrieve the
key from Identity.

Add Certificate issuer, getName and revoke

Throw an exception if GnuTLS is too old
instead of not providing the API.

Also consistently throw in case of hash error,
and return 0 for an empty hash, for both
PublicKey and Certificate.

Generate random hash in 32 bits chunks
instead of 8 bits chunks.

dht: fix searchstep not scheduled after announcing

for now just print an error

pht: prevent unneeded use of shared_ptr

Prefer a const reference instead of shared_ptr if the shared_ptr
is not used and argument type is not changed in the method.

This allows to prevent useless shared_ptr allocations.

In getRealPrefix, use OpState to track state instead of using many
different shared_ptr.

FieldSelectorDescription is just Value::Field and
adds nothing on top of it.

Remove it in favor of Value::Field.

argon2: use as submodule (reverted from commit c424eb91) (reverted from commit 3c45e199)

benchmark: fix invalid ifname error

fixes the contrib paths for the Visual Studio build