emacs: add notmuch-show-worker function for specifying crypto processing directly

The main reason to introduce this new unexposed function is to allow
the buffer redisplay crypto switch to behaving in a more expected way.
The prefix to notmuch-show-redisplay buffer now switches the crypto
processing of the current show buffer, as opposed to switching the
logic of the notmuch-crypto-process-mime customization variable.  This
behavior is more intuitive.