quota: resolve data race warnings/bugs reported by go test -race
quota: resolve data race warnings/bugs reported by go test -race

one of the report stacktraces:

```
==================
WARNING: DATA RACE
Write at 0x00c0212813e0 by goroutine 28:
  github.com/go-graphite/go-carbon/carbonserver.(*trieIndex).applyQuotas()
      /Users/xhu/go/src/github.com/go-graphite/go-carbon/carbonserver/trie.go:1770 +0x580
  github.com/go-graphite/go-carbon/carbonserver.TestTrieQuotaConcurrentApplyAndEnforce.func2()
      /Users/xhu/go/src/github.com/go-graphite/go-carbon/carbonserver/trie_test.go:1500 +0x278

Previous read at 0x00c0212813e0 by goroutine 20:
  github.com/go-graphite/go-carbon/carbonserver.(*trieIndex).throttle()
      /Users/xhu/go/src/github.com/go-graphite/go-carbon/carbonserver/trie.go:2156 +0x228
  github.com/go-graphite/go-carbon/carbonserver.TestTrieQuotaConcurrentApplyAndEnforce()
      /Users/xhu/go/src/github.com/go-graphite/go-carbon/carbonserver/trie_test.go:1525 +0x5e4
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.18.1/libexec/src/testing/testing.go:1439 +0x18c
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.18.1/libexec/src/testing/testing.go:1486 +0x44
```

deniszh/go-carbon,deniszh/go-carbon,deniszh/go-carbon