Only reset AES key in reject() if include_accepted

This is a mitigation for issue 8263, so that if multiple
'normal' rejections (of pre keys) are done quickly or
concurrently, the buggy handling of key updates
is not encountered.
Wait for new time_t before updating AES key file

This is a mitigation for issue 8263, wherein the master
experiences a communications failure when multiple
key updates happen in one second.  We reduce the chance
of this by checking time() against the mtime of the file
and waiting until they are not equal.