Refactor for session management (#193)

Previously every Handler function was receiving the session token in the
form of a jwt string, in consequence every time we want to access the
encrypted claims of the jwt we needed to run a decryption process,
additionally we were decrypting the jwt twice, first at the session
validation then inside each handler function, this was also causing a
lot of using related to the merge between m3 and mcs

What changed:

Now we validate and decrypt the jwt once in `configure_mcs.go`, this
works for both, mcs (console) and operator sessions, and then pass the
decrypted claims to all the functions that need it, so no further token
validation or decryption is need it.

pkg/auth/jwt.go

@@ -32,6 +32,7 @@ import (
 
 	jwtgo "github.com/dgrijalva/jwt-go"
 	"github.com/go-openapi/swag"
+	"github.com/minio/mcs/models"
 	xjwt "github.com/minio/mcs/pkg/auth/jwt"
 	"github.com/minio/minio-go/v6/pkg/credentials"
 	uuid "github.com/satori/go.uuid"
@@ -210,7 +211,7 @@ func GetTokenFromRequest(r *http.Request) (*string, error) {
 	return swag.String(reqToken), nil
 }
 
-func GetClaimsFromTokenInRequest(req *http.Request) (*DecryptedClaims, error) {
+func GetClaimsFromTokenInRequest(req *http.Request) (*models.Principal, error) {
 	sessionID, err := GetTokenFromRequest(req)
 	if err != nil {
 		return nil, err
@@ -221,5 +222,10 @@ func GetClaimsFromTokenInRequest(req *http.Request) (*DecryptedClaims, error) {
 	if err != nil {
 		return nil, err
 	}
-	return claims, nil
+	return &models.Principal{
+		AccessKeyID:     claims.AccessKeyID,
+		Actions:         claims.Actions,
+		SecretAccessKey: claims.SecretAccessKey,
+		SessionToken:    claims.SessionToken,
+	}, nil
 }