gpg: Disable keydb handle caching

* g10/getkey.c (getkey_end): Disable caching of the open keydb
handle.

This created a big regression for Windows because the keyring
is only released after the global ctrl is released. So if an operation
does a getkey and then tries to modify the keyring it will fail on
Windows with a sharing violation. We need to modify all
keyring write operations to also take the ctrl and close the
cached_getkey_kdb handle to make writing work. See:
https://dev.gnupg.org/T3097

• GnuPG-Bug-Id: T3097
• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>