emacs: notmuch search bugfix

The recent change to use json for notmuch-search.el introduced a bug
in the code for keeping position on refresh. The problem is a
comparison between (plist-get result :thread) and a thread-id returned
by notmuch-search-find-thread-id: the latter is prefixed with
"thread:"

We fix this by adding an option to notmuch-search-find-thread-id to
return the bare thread-id. It appears that notmuch-search-refresh-view
is the only caller of notmuch-search that supplies a thread-id so this
change should be safe (but could theoretically break users .emacs
functions).