Keep track of number of times search cache is cleared.

Allows external consumers of the db to maintain their own search caches.

Also make last_modified() a first class API.

src/calibre/db/cache.py

@@ -107,6 +107,7 @@ class Cache(object):
         self.dirtied_cache = {}
         self.dirtied_sequence = 0
         self.cover_caches = set()
+        self.clear_search_cache_count = 0
 
         # Implement locking for all simple read/write API methods
         # An unlocked version of the method is stored with the name starting
@@ -189,8 +190,13 @@ class Cache(object):
 
     @write_api
     def clear_search_caches(self, book_ids=None):
+        self.clear_search_cache_count += 1
         self._search_api.update_or_clear(self, book_ids)
 
+    @read_api
+    def last_modified(self):
+        return self.backend.last_modified()
+
     @write_api
     def clear_caches(self, book_ids=None, template_cache=True, search_cache=True):
         if template_cache:

src/calibre/db/legacy.py

@@ -121,7 +121,7 @@ class LibraryDatabase(object):
         return self.backend.dbpath
 
     def last_modified(self):
-        return self.backend.last_modified()
+        return self.new_api.last_modified()
 
     def check_if_modified(self):
         if self.last_modified() > self.last_update_check: