Multisegment download support

[pandomic]

It is currently impossible to perform multisegmented downloads.

Sometimes we may have multiple downloadable segments coming from EBICS. In this case, the first request/response pair is transferred in Initialisation phase. Following requests are delivered in Transfer phase and may not contain a transaction key. After all segments have been transferred, the client may request a Receipt.

Diagram from spec:

Proposed PR contains a loop that makes requests until the last segment is received from EBICS.