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858803cc2c
The pool usage was kind of awkward previously: if the user of a pool
decided that a previously borrowed object should no longer be used,
it was their responsibility to destroy the object (releasing associated
resources and so on) and then call `steal()` on the pool to free space
for a new object.
Change the interface. Now the `Pool` constructor obtains a `destroy`
function additionally to the `build` function. The user calls the
function `put` to return both objects that are still usable and those
aren't. For the latter, they set `is_dirty=True`. The pool will
'destroy' the object with the provided function, which could mean e.g.
releasing associated resources.
For example, instead of:
```
if self.cluster.is_dirty:
self.clusters.stop()
self.clusters.release_ips()
self.clusters.steal()
else:
self.clusters.put(self.cluster)
```
we can now use:
```
self.clusters.put(self.cluster, is_dirty=self.cluster.is_dirty)
```
(assuming that `self.clusters` is a pool constructed with a `destroy`
function that stops the cluster and releases its IPs.)
Also extend the interface of the context manager obtained by
`instance()` - the user must now pass a flag `dirty_on_exception`. If
the context manager exists due to an exception and that flag was `True`,
the object will be considered dirty. The dirty flag can also be set
manually on the context manager. For example:
```
async with (cm := pool.instance(dirty_on_exception=True)) as server:
cm.dirty = await run_test(test, server)
# It will also be considered dirty if run_test throws an exception
```