schemata: Disambiguate non-terminal names

Previously, the show schema and the search schema used different
"thread" non-terminals.  While these schemata don't interact, this is
still confusing, so rename search's "thread" to "thread_summary".  To
further limit confusion, prefix all top-level search non-terminals now
begin with "search_".

devel/schemata

@@ -122,21 +122,21 @@ notmuch search schema
 ---------------------
 
 # --output=summary
-summary = [thread*]
+search_summary = [thread_summary*]
 
 # --output=threads
-threads = [threadid*]
+search_threads = [threadid*]
 
 # --output=messages
-messages = [messageid*]
+search_messages = [messageid*]
 
 # --output=files
-files = [string*]
+search_files = [string*]
 
 # --output=tags
-tags = [string*]
+search_tags = [string*]
 
-thread = {
+thread_summary = {
     thread:         threadid,
     timestamp:      unix_time,
     date_relative:  string,   # user-friendly timestamp