Better handling of B2 authorization failures

This commit fixed 2 issues wrt Backblaze B2 authorization:
* every thread may call b2_authorize_account at the same time when there
are 401 errors
* if B2 has a login outage, then all threads will call b2_authorize_account
repeatedly without delay

A simple solution is to limit one b2_authorize_account call to once every
30 second regardless of how many threads there are.  If the call to
b2_authorize_account is not allowed, the random exponential backoff will
be performed.

src/duplicacy_b2client_test.go

@@ -50,7 +50,7 @@ func TestB2Client(t *testing.T) {
 
 	b2Client.TestMode = true
 
-	err := b2Client.AuthorizeAccount(0)
+	err, _ := b2Client.AuthorizeAccount(0)
 	if err != nil {
 		t.Errorf("Failed to authorize the b2 account: %v", err)
 		return