Fuzz testing the JSONRPC server

[darosior]

As it's exposed in a single server module, it think we could fuzz the mio_loop: https://github.com/re-vault/revaultd/blob/3141908c8c1e847b2e76f9240c109734d1324599/src/daemon/jsonrpc/server.rs#L121-L125

We could use a custom MetaIoHandler that would not process the actual command but rather be used to acknowledge their reception. This would largely complement our unit tests that are only for now testing for simple situations and edge cases that we uncover as we go.

We could use the great libFuzzer integration by the rust-fuzz org.