Improve problem access waiting

[randomir]

Here we:

• fix In Windows OS, terminal hangs if I try to show a sampleset a second time #104 by disabling caching of problem access callback response, so that each problem load notifies the server, even for repeated loads of agent-cached problems;
• introduce timeout parameter in dwave.inspector.show() that works in conjunction with the block param to determine max wait time of show() for problem access.

[JoelPasvolsky]

This solves problem #104 on unix but WIN still hangs (with or without explicit timeout:

In [5]: sampleset = EmbeddingComposite(DWaveSampler()).sample(bqm, num_reads=100)

In [6]: dwave.inspector.show(sampleset, timeout=1)
Out[6]: 'http://127.0.0.1:18000/?problemId=ccae2bd8-3c0d-4271-b63c-8afce436602d'

In [7]: dwave.inspector.show(sampleset, timeout=1)

[randomir]

It hangs even with timeout set?! Hm..

[randomir]

For posterity, @JoelPasvolsky says this PR fixes #104, but he's still seeing #48.