Add support for extending existing postage stamp TTL

[agazso]

This could be a new mode besides the existing two modes (namely using a predefined postage stamp or buying new postage stamps when they expire). The node would check the existing postage stamp TTL and when reaching a threshold value, instead of buying a new postage stamp it would extend the TTL of the existing stamp using the top-up endpoint.

An extra feature related to this could be to extend the capacity (depth) of an existing postage stamp (see #311).

[agazso]

The feature should extend all valid postage stamps that are about to expire. Valid postage stamps are those that hasn't expired. If there is no valid postage stamp it should buy one for the same period as the extension period (just like with the autobuy feature).