oauth2, oauth2/google: add, use ReuseTokenSource

Token caching is now done whenever you make a Client, and
ReuseTokenSource is exported from the oauth2 package and used by the
Google TokenSources (Compute and App Engine).

Token.Expired is now Token.Valid, and works on nil receivers.

Some other wording cleanups in the process.

All tests pass. App Engine should pass, but is untested.

Change-Id: Ibe1d2599ac3ccfe9b399b1672f74bb24cfc8d311
Reviewed-on: https://go-review.googlesource.com/2195
Reviewed-by: Burcu Dogan <jbd@google.com>

jwt.go

@@ -52,33 +52,21 @@ type JWTConfig struct {
 
 // TokenSource returns a JWT TokenSource using the configuration
 // in c and the HTTP client from the provided context.
-//
-// The returned TokenSource only does JWT requests when necessary but
-// otherwise returns the same token repeatedly until it expires.
-//
-// The provided initialToken may be nil, in which case the first
-// call to TokenSource will do a new JWT request.
-func (c *JWTConfig) TokenSource(ctx Context, initialToken *Token) TokenSource {
-	return &newWhenNeededSource{
-		t:   initialToken,
-		new: jwtSource{ctx, c},
-	}
+func (c *JWTConfig) TokenSource(ctx Context) TokenSource {
+	return ReuseTokenSource(nil, jwtSource{ctx, c})
 }
 
 // Client returns an HTTP client wrapping the context's
 // HTTP transport and adding Authorization headers with tokens
 // obtained from c.
 //
-// The provided initialToken may be nil, in which case the first
-// call to TokenSource will do a new JWT request.
-//
 // The returned client and its Transport should not be modified.
-func (c *JWTConfig) Client(ctx Context, initialToken *Token) *http.Client {
-	return NewClient(ctx, c.TokenSource(ctx, initialToken))
+func (c *JWTConfig) Client(ctx Context) *http.Client {
+	return NewClient(ctx, c.TokenSource(ctx))
 }
 
 // jwtSource is a source that always does a signed JWT request for a token.
-// It should typically be wrapped with a newWhenNeededSource.
+// It should typically be wrapped with a reuseTokenSource.
 type jwtSource struct {
 	ctx  Context
 	conf *JWTConfig