Update numeric store getters to return bool if value is missing

[robert-zaremba]

In Numeric getters (Big Int, Dec, int, int64...) returned zero value if the value was not present in the store, without letting know if the value is there. That can cause unexpected behavior when zero is a proper value, but we want to know if it was present in the store or not.

This PR adds bool return type to numeric getters and the time getter.

[codecov]

Codecov Report

Merging #2169 (0863e9f) into main (7f05ad4) will decrease coverage by 5.09%.
Report is 153 commits behind head on main.
The diff coverage is 81.17%.

❗ Current head 0863e9f differs from pull request most recent head f8bfd36. Consider uploading reports for the commit f8bfd36 to get more accurate results

@@            Coverage Diff             @@
##             main    #2169      +/-   ##
==========================================
- Coverage   75.38%   70.30%   -5.09%     
==========================================
  Files         100      148      +48     
  Lines        8025    11310    +3285     
==========================================
+ Hits         6050     7951    +1901     
- Misses       1589     2816    +1227     
- Partials      386      543     +157     

Files Changed	Coverage Δ
ante/spam_prevention.go	75.92% <ø> (ø)
x/incentive/codec.go	47.82% <ø> (+9.89%)	⬆️
x/incentive/keeper/invariants.go	0.00% <0.00%> (ø)
x/incentive/keeper/unbonding.go	80.45% <ø> (ø)
x/incentive/keeper/update.go	52.11% <ø> (ø)
x/incentive/keys.go	100.00% <ø> (ø)
x/incentive/msgs.go	77.41% <ø> (-0.81%)	⬇️
x/incentive/params.go	89.28% <ø> (-10.72%)	⬇️
x/leverage/client/tests/suite.go	100.00% <ø> (ø)
x/leverage/client/tests/tests.go	100.00% <ø> (ø)
... and 54 more

... and 54 files with indirect coverage changes