feat(v2): read path improvements (#3675)

* use go-concurrency-limits to handle query backend concurrency

* Refactor query plan

* Wrap go-concurrency-limits, add test

Author: aleks-p

.golangci.yml

@@ -72,6 +72,8 @@ issues:
     - Error return value of .*log\.Logger\)\.Log\x60 is not checked
     - Error return value of .*.Log.* is not checked
     - Error return value of `` is not checked
+    # See https://github.com/grpc/grpc-go/issues/7090 for more information
+    - grpc.Dial(.*) is deprecated
 
   # which dirs to skip: they won't be analyzed;
   # can use regexp here: generated.*, regexp is applied on full path;