fix worker iteration for a tree index

Before the patch, during the worker iteration (in case of using
a tree index) an array of tuples for check on expiration was
obtained before suspending, and some tuples can be modified / deleted
from another fiber while the worker fiber is sleeping.
Now an array of tuples for check on expiration obtained just before
checking.

Author: FizikRoot

expirationd.lua

@@ -94,9 +94,9 @@ local function tree_index_iter(scan_space, task)
         for _, tuple in ipairs(tuples) do
             expiration_process(task, tuple)
         end
+        suspend(scan_space, task)
         local key = construct_key(scan_space.id, last_id)
         tuples = scan_space.index[0]:select(key, params)
-        suspend(scan_space, task)
     end
 
 end