Use one dirmngr instance for multiple lists / handle key retrieval from keyservers on our own
As discussed in #289 (closed), currently, dirmngr uses a huge amount of memory, and we need several instances, one per lists. On busy servers with many lists, this quickly exhausts memory. A sane way would be to use of dirmngr instance for multiple, or all, lists.
This needs work upstream, see this ticket: https://dev.gnupg.org/T3767