Thread-safe client close

[randomir]

A follow-up to #680, in which we make sure client use is disabled also during closing, not just after the client has been closed.

We also make sure close() is thread-safe wrt attempted usage while closing.

TODO: consider converting Client.solvers_session to a ctxmgr, or making this session ephemeral in another way.
➡️ Keeping Client.solvers_session active during client lifespan, as it saves ~3ms on init due to disk cache access.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.13%. Comparing base (59f8baa) to head (f506232).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #682      +/-   ##
==========================================
+ Coverage   89.10%   89.13%   +0.02%     
==========================================
  Files          44       44              
  Lines        4846     4850       +4     
==========================================
+ Hits         4318     4323       +5     
+ Misses        528      527       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[arcondello]

LGTM!

I suppose if you also wanted a context manager you could do something with a classmethod. Maybe

class Client:
    @contextlib.contextmanager
    @classmethod
    def as_ctx(cls, *args, **kwargs):
        client = cls(*args, **kwargs)
        yield client
        client.close()

might be less performant but convenient in some cases?

[randomir]

@arcondello, Client already is a context manager (see __enter__ and __exit__ methods).

I considered converting solvers_session property to a context manager, but decided not to do it for now to save some init time.