NTS Pool: add KeepAlive

[folkertdev]

according to the NTS specification, the TLS connection must be closed after an EndOfMessage record. Hence we have to define different behavior for the records added for the NTS Pool KE. This is important for the connection that asks a server for its supported algorithms (that we then want to re-use for actually getting cookies). And also in general if we want to keep the connection to a NTS KE server open so we don' t repeat (relatively expensive) TLS handshakes.