Improve sled-based light store

[romac]

Close: #769

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[codecov-io]

Codecov Report

Merging #773 (cda64a8) into master (97532b3) will increase coverage by 3.3%.
The diff coverage is 62.0%.

@@           Coverage Diff            @@
##           master    #773     +/-   ##
========================================
+ Coverage    50.4%   53.8%   +3.3%     
========================================
  Files         197     197             
  Lines       13607   13682     +75     
  Branches     3266    3284     +18     
========================================
+ Hits         6870    7369    +499     
+ Misses       6475    5946    -529     
- Partials      262     367    +105     

Impacted Files	Coverage Δ
light-client/src/builder/light_client.rs	0.0% <ø> (ø)
light-client/src/components/scheduler.rs	0.0% <0.0%> (ø)
light-client/src/light_client.rs	57.8% <ø> (ø)
light-client/src/supervisor.rs	28.6% <0.0%> (ø)
light-client/src/utils/std_ext.rs	31.2% <0.0%> (-14.3%)	⬇️
light-node/src/commands/initialize.rs	0.0% <0.0%> (ø)
light-node/src/commands/start.rs	0.0% <0.0%> (ø)
light-client/src/store/memory.rs	35.4% <14.2%> (-5.1%)	⬇️
light-client/src/store.rs	40.6% <30.0%> (-11.4%)	⬇️
light-client/src/store/sled/utils.rs	55.3% <70.2%> (+55.3%)	⬆️
... and 62 more

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[romac]

Note: LightStore::lowest_trusted_or_verified is not currently used but will be useful for improving performance of backward verification.

[thanethomson]

Looks great! 👍