Implement conditional problem submit

Initially, the only condition can be a time limit. On problem submit, max allotted wall clock time can be given as additional parameter, e.g.:
```
solver.sample_ising(h, J, timeout=100)
```

In the first implementation, we'll have to track timeout on client side. But in future, SAPI could support conditions on submit endpoint. Something along the lines of `X-If-Retrievable-Until` header field.