tpc key validity should not be rechecked after the initial validation

[costing]

The TPC key is checked for validity at every connect / reconnect attempt. For long transfers this seems to cause problems since it expires by the time the client reconnects after a timeout in reading. @abh3 suggests that after the initial checking of the tpc key, the same key should be accepted in subsequent attempts from the same host without rechecking the expiration time.