Adds cached result backend

[Koed00]

• Adds a global and per-task ``cached` option, which will redirect any results to the cache backend.
• Adds count option for groups results which will block until the indicated amount of results in the group has been reached
• Adds wait option for group results. Can be used in conjunction with count
• Adds async_iter() command which takes an iterable set of arguments, asyncs them individualy against the cache backend and collates them back in to a single database result.

[codecov-io]

Current coverage is 96.15%

Merging #84 into master will increase coverage by +0.32% as of 0fc9434

@@            master     #84   diff @@
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  Files           37      38     +1
  Stmts         2162    2416   +254
  Branches         0       0       
  Methods          0       0       
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+ Hit           2072    2323   +251
  Partial          0       0       
- Missed          90      93     +3

Review entire Coverage Diff as of 0fc9434

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