crud: store the cursor's after_tuple explicitly as a Lua table

In scope of tarantool/tarantool#8147, a new context-dependent extension for
box tuples, `MP_TUPLE`, is introduced. If the merger's buffer source is
used, raw MsgPack is received, which does not allow for passing the context
required for decoding `MP_TUPLE`.

At the same time, all box tuples are now encoded as `MP_TUPLE` by default.
If the merger's tuple chunk can be decoded by manually skipping the
extension header of `MP_TUPLE` and the tuple format of individual tuples,
the cursor is decoded using the Lua `msgpack` library, which cannot handle
`MP_TUPLE`s without a decoding context.

The cursor's `after_tuple` field is a box tuple, so in order to overcome
the limitation above, we need to explicitly convert it to a Lua table.

Needed for tarantool/tarantool#8147

Author: CuriousGeorgiy

crud/readview.lua

@@ -168,7 +168,7 @@ local function select_readview_on_storage(space_name, index_id, conditions, opts
         cursor.is_end = true
     else
         local last_tuple = resp.tuples[#resp.tuples]
-        cursor.after_tuple = last_tuple
+        cursor.after_tuple = last_tuple:totable()
     end
 
     cursor.stats = {

crud/select.lua

@@ -99,7 +99,7 @@ local function select_on_storage(space_name, index_id, conditions, opts)
         cursor.is_end = true
     else
         local last_tuple = resp.tuples[#resp.tuples]
-        cursor.after_tuple = last_tuple
+        cursor.after_tuple = last_tuple:totable()
     end
 
     cursor.stats = {