gpg: Allow generation of more than 4096 keys in one run.

* g10/getkey.c (cache_public_key): Make room in the cache if needed.

To create the selfsigs, the key generation code makes use of the key
cache. However, after 4096 the cache is filled up and then disabled.
Thus generating more than 4096 keys in one run was not possible. We
now clear the first half the inserted keys every time the cache gets
full.