Cache secrets and concurrent decryption (#790)

Related to #782 and #444 

- Allows concurrent decryption of different secrets files
- Caches decrypted secrets by original file path and returns decrypted results from memory
- Secrets being run through an instance of helmexec will be cached and run as fast as possible concurrently

NB: This particular PR doesn't make _all_ calls to secrets cached and concurrent.  Environment Secrets in particular seem to not be evaluated with a ScatterGather(), and doesn't use the same helmexec instance as other parts of the code, so it doesn't take advantage of these changes.  Some reworking of the plumbing there would be needed.

pkg/helmexec/exec_test.go

@@ -228,10 +228,16 @@ func Test_DecryptSecret(t *testing.T) {
 	if err != nil {
 		t.Errorf("Error: %v", err)
 	}
-	expected := fmt.Sprintf(`Decrypting secret %s/secretName
+	// Run again for caching
+	helm.DecryptSecret(HelmContext{}, "secretName")
+
+	expected := fmt.Sprintf(`Preparing to decrypt secret %v/secretName
+Decrypting secret %s/secretName
 exec: helm secrets dec %s/secretName --kube-context dev
 exec: helm secrets dec %s/secretName --kube-context dev: 
-`, cwd, cwd, cwd)
+Preparing to decrypt secret %s/secretName
+Found secret in cache %s/secretName
+`, cwd, cwd, cwd, cwd, cwd, cwd)
 	if buffer.String() != expected {
 		t.Errorf("helmexec.DecryptSecret()\nactual = %v\nexpect = %v", buffer.String(), expected)
 	}