Between picking an LSM tree from a heap and taking a reference to it in
vy_task_new() there are a few places where the scheduler may yield:
 - in vy_worker_pool_get() to start a worker pool;
 - in vy_task_dump_new() to wait for a memory tree to be unpinned;
 - in vy_task_compaction_new() to commit an entry to the metadata log
   after splitting or coalescing a range.

If a concurrent fiber drops and deletes the LSM tree in the meanwhile,
the scheduler will crash. To avoid that, let's take a reference to
the LSM tree.

It's quite difficult to write a functional test for it without a bunch
of ugly error injections so we rely on fuzzing tests.

Closes #9995

NO_DOC=bug fix
NO_TEST=fuzzing