utubettl: fix slow take on busy utubes

If some of the utubettl for tasks at the top of the queue were busy
most of the time, `take` would slow down for every other task.
This problem is fixed by creating a new space `space_ready`. It
contains first task with `READY` status from each utube.

This solution shows great results for the stated problem, with the cost
of slowing the `put` method (it is ~3 times slower). Thus, this workaround is
disabled by default. To enable it, user should set the
`storage_mode = "utube_ready_space"` as an option while creating the tube.
As example:
```lua
local test_queue = queue.create_tube('test_queue', 'utubettl',
        {temporary = true, storage_mode = "utube_ready_space"})
```

Closes #228

CHANGELOG.md

@@ -8,16 +8,16 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ## [Unreleased]
 
 ### Added
-- `storage_mode` option for creating a `utube` tube (#228). It enables the
-  workaround for slow takes while working with busy tubes.
+- `storage_mode` option for creating a `utube` and `utubettl` tube (#228).
+  It enables the workaround for slow takes while working with busy tubes.
 
 ### Fixed
 
 - Stuck in `INIT` state if an instance failed to enter the `running` mode
   in time (#226). This fix works only for Tarantool versions >= 2.10.0.
-- Slow takes on busy `utube` tubes (#228). The workaround could be enabled by
-  passing the `storage_mode = "utube_ready_space"` option while creating
-  the tube.
+- Slow takes on busy `utube` and `utubettl` tubes (#228). The workaround could
+  be enabled by passing the `storage_mode = "utube_ready_space"` option while
+  creating the tube.
 
 ## [1.3.3] - 2023-09-13
 

README.md

@@ -220,6 +220,11 @@ on disk
 already exists
   * `on_task_change` - function name - a callback to be executed on every
 operation
+  * `storage_mode` - string - one of
+      * `"default"` - default implementation of `utube`
+      * `"utube_ready_space"` - allows processing `take` requests faster, but
+        by the cost of `put` operations speed. Right now this option is enabled
+        only for `memtx` engine
 
 The following options can be specified when putting a task in a
 `utubettl` queue: