Offchain RPC methods should not SCALE-encode keys & values

[vgeddes]

The Offchain DB should be treated as a generic key-value store, taking raw bytes for keys & values. The current implementation prevents us from reading values for arbitrary keys.

[codecov]

Codecov Report

Merging #171 (3fe6e43) into master (db7f2af) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #171   +/-   ##
=======================================
  Coverage   58.51%   58.52%           
=======================================
  Files          98       98           
  Lines        3611     3602    -9     
=======================================
- Hits         2113     2108    -5     
+ Misses       1081     1079    -2     
+ Partials      417      415    -2     

Impacted Files	Coverage Δ
rpc/offchain/get_local_storage.go	62.50% <100.00%> (+10.50%)	⬆️
rpc/author/submit_and_watch_extrinsic.go	72.22% <0.00%> (-11.12%)	⬇️

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[vedhavyas]

LGTM

Thanks!

[vedhavyas]

more here - https://github.com/paritytech/substrate/blob/master/client/offchain/src/api.rs#L87