Disable background decoding for images from the in memory cache.

[ghost]

• this improves the performance of image loading for in-memory cached
  images.

We observe a flicker when returing to an image view that renders
multiple images using kf_setImageWithResource. We traced through the
calls and found a place where Kingfisher attempts to re-decode an image
that has already been decoded and stored in the in memory cache.

We think that it is safe to pull out an image from the in memory cache and not
attempt to decode again.

[codecov-io]

Current coverage is 77.28%

Merging #257 into master will increase coverage by +0.38% as of 718e050

@@            master    #257   diff @@
======================================
  Files           11      11       
  Stmts         1260    1250    -10
  Branches         0       0       
  Methods          0       0       
======================================
- Hit            969     966     -3
  Partial          0       0       
+ Missed         291     284     -7

Review entire Coverage Diff as of 718e050

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[onevcat]

Yes, you are right.