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_b2storage.go

@@ -19,7 +19,7 @@ func CreateB2Storage(accountID string, applicationKey string, bucket string, sto
 
 	client := NewB2Client(accountID, applicationKey, storageDir, threads)
 
-	err = client.AuthorizeAccount(0)
+	err, _ = client.AuthorizeAccount(0)
 	if err != nil {
 		return nil, err
 	}