Restore thread search behavior

Restore thread search behavior

* lisp/gnus/gnus-search.el (gnus-search-thread): Make this function
  produce an engine-agnostic search query.
  (gnus-search-prepare-query): Fix dumb error.
  (gnus-search-indexed-search-command): Edit to handle the 'thread
  key.
  (gnus-search-run-search): In thread searches, have the imap
  implementation expand Message-Id searches to include the References
  header. Also, somewhere along the way we lost the
  `gnus-search-get-active' call.
  (gnus-search-run-search): For Notmuch, add an :around method on this
  function, which does a primary search for thread-ids, then passes
  off to the secondary search for the messages themselves.
  (gnus-search-transform-expression): Forgot
  that multiple nested ORs have to be parenthesized for IMAP.
* lisp/gnus/nnselect.el (nnselect-request-thread): Alter function to
  pass in a generic thread search query; no longer calls imap-specific
  code.