authenticate: make service http only

- Rename SessionState to State to avoid stutter.
- Simplified option validation to use a wrapper function for base64 secrets.
- Removed authenticates grpc code.
- Abstracted logic to load and validate a user's authenticate session.
- Removed instances of url.Parse in favor of urlutil's version.
- proxy: replaces grpc refresh logic with forced deadline advancement.
- internal/sessions: remove rest store; parse authorize header as part of session store.
- proxy: refactor request signer
- sessions: remove extend deadline (fixes #294)
- remove AuthenticateInternalAddr
- remove AuthenticateInternalAddrString
- omit type tag.Key from declaration of vars TagKey* it will be inferred
  from the right-hand side
- remove compatibility package xerrors
- use cloned http.DefaultTransport as base transport

proxy/proxy_test.go

@@ -169,9 +169,6 @@ func TestOptions_Validate(t *testing.T) {
 	authurl, _ := url.Parse("authenticate.corp.beyondperimeter.com")
 	authenticateBadScheme := testOptions(t)
 	authenticateBadScheme.AuthenticateURL = authurl
-	authenticateInternalBadScheme := testOptions(t)
-	authenticateInternalBadScheme.AuthenticateInternalAddr = authurl
-
 	authorizeBadSCheme := testOptions(t)
 	authorizeBadSCheme.AuthorizeURL = authurl
 	authorizeNil := testOptions(t)
@@ -200,7 +197,6 @@ func TestOptions_Validate(t *testing.T) {
 		{"nil options", config.Options{}, true},
 		{"authenticate service url", badAuthURL, true},
 		{"authenticate service url no scheme", authenticateBadScheme, true},
-		{"internal authenticate service url no scheme", authenticateInternalBadScheme, true},
 		{"authorize service url no scheme", authorizeBadSCheme, true},
 		{"authorize service cannot be nil", authorizeNil, true},
 		{"no cookie secret", emptyCookieSecret, true},
@@ -221,7 +217,6 @@ func TestOptions_Validate(t *testing.T) {
 }
 
 func TestNew(t *testing.T) {
-
 	good := testOptions(t)
 	shortCookieLength := testOptions(t)
 	shortCookieLength.CookieSecret = "gN3xnvfsAwfCXxnJorGLKUG4l2wC8sS8nfLMhcStPg=="