Move the socket file to a specific directory for which the node will
use its data-dir and make the validator able to communicate through
stdin/stdout allowing a standalone usage via a command-line option.
Also contains minor UX improvements.

Fixes #761

This commit does the following:

1/ It fixes disk leaks in the mockup's tests. Indeed
   each successful execution of the mockup's tests was
   leaving **9** dangling directories named /tmp/tezos-client*
2/ It gets rid of client.py's set_base_dir method
   which relies on a side effect and hence makes
   mockup initialization harder to understand.
3/ Simplifies initialization in tests of the mockup
   It was cumbersome for bad reasons before, mostly
   because it was authored by two tezos noobs (one of them
   being my earlier myself)

This commit removes the test
that `tezos-client --base-dir /tmp/file create mockup`
fails when /tmp/file is a file. This cannot be done anymore because
client.py's Client class defensively checks that the base directory is
a directory so an exception is raised even before calling `create mockup`
It's okay, it's not a super important test.

This is a suggestion from Arvid and is orthogonal to the topic
of this MR. It could have been done in earlier mockup MRs.

This is done by parameterizing the mockup_client fixture by the
two protocols, as well as parameterizing the tests that do
not use the fixture.

On the way, I moved the mockup_client fixture to conftest.py
as it doesn't hardcode the protocol anymore (this was the blocking point
for making it widely available).

We specify `--protocol` because it is required for `config show mockup`
and `config init mockup` to work. Previously it was working
by luck, because when `--protocol` is not specified, the client
will take the first proxy implementation, which happened to be alpha.
So it was working until we started testing carthage.

- Cleaner implementation
- Better support for signals
- Sounder and more versatile interface
- Tests

There was a window between the removal of a point from the incoming
table and the time where the file descriptor was closed or accounted
in active connection.
Accumulation of incoming connection in this window would lead to
exhaustion of available file descriptor and to node's death.

We also shouldn't crash the node because an exception arised in the
handling process of a specific connection.

Detached process results were not available from the main process,
this patch gives access to the detached functions results when the
process is not killed or canceled.

It also bring the possibility to use data-encoding for exchanges with
the detached process.

As it is not yet possible to define a parallel error that would relies
on the registration of recursive errors, the head of each error trace
is taken in the trace when joining multiple process.

Nesting of Lwt_main.run is forbidden since Lwt 5.
We used this nesting to prevent the yielded promises to be rescheduled
in the detached process, which results a kind of fork-bomb.

As it is not possible anymore, we have to run the forks sequentially
to ensure that there is no yielded promises waiting to fork inside the
forked process.

Add enclosing boxes, remove '\n' (use @, break hint) to get a nicer
pretty-printing of values for mockup constants.