gpg: Fix regression in --refresh-keys

* g10/keyserver.c (keyserver_get): Factor all code out to ...
(keyserver_get_chunk): new.  Extimate line length.
(keyserver_get): Split up requests into chunks.

Note that refreshing all keys still requires way to much memory
because we build an in-memory list of all keys first. It is required
to first get a list of all keys to avoid conflicts while updating the
key store in the process of receiving keys. A better strategy would
be a background process and tracking the last update in the key store.

• GnuPG-bug-id: T1755
• Signed-off-by: Werner Koch <wk@gnupg.org>