shield: implement distribution params

[yoongbok-lee]

Closes: #379
Related: #XXX

Description

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[xiumingdou]

Logically LGTM.
Seems test case TestBeginBlocker panicked.
=== RUN TestBeginBlocker/normal --- FAIL: TestBeginBlocker (0.04s) --- FAIL: TestBeginBlocker/normal (0.00s) panic: invalid coin denominations; stake, uctk [recovered] panic: invalid coin denominations; stake, uctk

[xiumingdou]

Just curious, how do we test upgrade?

[eRuaro]

Looks good to me!

[codecov]

Codecov Report

Merging #380 (308ca1a) into master (1ff5002) will increase coverage by 1.36%.
The diff coverage is 26.31%.

❗ Current head 308ca1a differs from pull request most recent head b5cd354. Consider uploading reports for the commit b5cd354 to get more accurate results

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
+ Coverage   47.46%   48.83%   +1.36%     
==========================================
  Files         102      110       +8     
  Lines        8258     8550     +292     
==========================================
+ Hits         3920     4175     +255     
+ Misses       3927     3921       -6     
- Partials      411      454      +43     

Impacted Files	Coverage Δ
app/config.go	0.00% <0.00%> (ø)
x/gov/keeper/proposal.go	42.75% <ø> (+3.44%)	⬆️
x/gov/keeper/vote.go	41.28% <ø> (-11.01%)	⬇️
x/mint/keeper/keeper.go	54.83% <0.00%> (ø)
x/shield/keeper/grpc_query.go	0.00% <0.00%> (ø)
x/shield/keeper/querier.go	0.45% <0.00%> (-0.03%)	⬇️
x/shield/keeper/rewards.go	0.00% <0.00%> (ø)
x/gov/keeper/deposit.go	61.76% <20.00%> (-9.67%)	⬇️
app/upgrade_handler.go	21.05% <33.33%> (+3.66%)	⬆️
x/shield/keeper/migrations.go	40.00% <40.00%> (ø)
... and 25 more