Bug 456910 - Stop using exclusive locking in places

This allows us to greatly reduce the number of fsyncs when upgrading to sqlite.
The perf loss in no longer using this locking is not significant (10^-6 %).
r=dietrich

toolkit/components/places/src/nsNavHistory.cpp

@@ -794,12 +794,6 @@ nsNavHistory::InitDB(PRInt16 *aMadeChanges)
   rv = mDBConn->ExecuteSimpleSQL(cacheSizePragma);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  // lock the db file
-  // http://www.sqlite.org/pragma.html#pragma_locking_mode
-  rv = mDBConn->ExecuteSimpleSQL(
-    NS_LITERAL_CSTRING("PRAGMA locking_mode = EXCLUSIVE"));
-  NS_ENSURE_SUCCESS(rv, rv);
-
   // moz_places
   if (!tableExists) {
     *aMadeChanges = DB_MIGRATION_CREATED;