##### SUMMARY
Replace the constant poll interval of 1 second, with a truncated
exponential back off algorithm with jitter.
Below is a suite of poll interval (in seconds) generated by the new
algorithm:
```
1.49
2.14
5.46
6.51
6.57
5.57
5.98
7.13
6.59
7.10
5.54
5.03
6.56
5.96
6.72
7.21
7.05
5.31
5.60
6.33
6.82
5.42
6.08
6.60
TOTAL: 140.77
```
##### SUMMARY
The class scoped `cache` dict was being shared across all
`cached_session`, we now make sure that the cache is instance scoped.
Fixes#403
##### ISSUE TYPE
- Bugfix Pull Request
