Ensure the caches always return Deferreds

[hawkowl]

This stores Deferreds consistently inside the cache.

[codecov]

Codecov Report

Merging #5583 into develop will decrease coverage by 0.5%.
The diff coverage is 86.36%.

@@             Coverage Diff             @@
##           develop    #5583      +/-   ##
===========================================
- Coverage    63.15%   62.64%   -0.51%     
===========================================
  Files          328      328              
  Lines        35901    35880      -21     
  Branches      5914     5907       -7     
===========================================
- Hits         22672    22477     -195     
- Misses       11604    11752     +148     
- Partials      1625     1651      +26

[codecov]

Codecov Report

Merging #5583 into develop will increase coverage by 0.02%.
The diff coverage is 66.66%.

@@             Coverage Diff             @@
##           develop    #5583      +/-   ##
===========================================
+ Coverage    63.15%   63.17%   +0.02%     
===========================================
  Files          328      328              
  Lines        35901    35896       -5     
  Branches      5914     5915       +1     
===========================================
+ Hits         22672    22679       +7     
+ Misses       11604    11590      -14     
- Partials      1625     1627       +2

[richvdh]

I think this is fixing #5582, among other things

[erikjohnston]

This does break the length stuff in lrucache, which I'll have to come back to (if it's actually useful?)

I believe we heavily rely on this feature to ensure that our cache memory usage is proportional to the cache size? Without it we'll end up with unpredictable memory usage.

[richvdh]

I fear this has bit-rotten beyond repair :/