Integrate gRPC queries into BaseApp

[aaronc]

ref: #5921

This PR:

• renames GRPCRouter to GRPCQueryRouter which makes more sense because it is just for queries (other non-query gRPC services would be handled separately)
• adds GRPCQueryRouter to BaseApp and integrates it into the ABCI query handler

---

For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[codecov]

Codecov Report

Merging #6335 into master will increase coverage by 0.03%.
The diff coverage is 54.71%.

@@            Coverage Diff             @@
##           master    #6335      +/-   ##
==========================================
+ Coverage   55.61%   55.65%   +0.03%     
==========================================
  Files         448      448              
  Lines       26947    26961      +14     
==========================================
+ Hits        14987    15005      +18     
+ Misses      10886    10879       -7     
- Partials     1074     1077       +3     

[odeke-em]

Thank you @aaronc, just a minor nit.

[fedekunze]

LGTM

[lgtm-com]

This pull request introduces 7 alerts when merging abaf39a into b1f483f - view on LGTM.com

new alerts:

• 5 for Expression has no effect
• 2 for Unreachable statement

[alexanderbez]

ACK :)