various: add golangci-lint, fix issues (#7905)

This adds an initial and intentionally minimal configuration for
golang-ci, fixes the issues reported, and adds a GitHub Action to check
new pull requests against this linter configuration.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I8f38fbc315836a19a094d0d3e986758b9313f163

tka/tailchonk_test.go

@@ -600,7 +600,7 @@ type compactingChonkFake struct {
 
 func (c *compactingChonkFake) AllAUMs() ([]AUMHash, error) {
 	out := make([]AUMHash, 0, len(c.Mem.aums))
-	for h, _ := range c.Mem.aums {
+	for h := range c.Mem.aums {
 		out = append(out, h)
 	}
 	return out, nil