citus_move_shard_placement() uses lots of memory on the coordinator

On an idle Citus cluster `citus_move_shard_placement()` to move shards from one worker to another (neither is the coordinator) peaks at 70 GB memory usage on the coordinator. In fact, we were unable to have single `citus_move_shard_placement()` complete on a non-idle cluster due to the OOM kills. Below is the memory usage as reported by Kubernetes while `citus_move_shard_placement()` was running.

This is somewhat surprising and looks like a leak since in this case the coordinator is not expected to take part in shard movement beyond orchestrating it.

Postgres version:
```
PostgreSQL 15.13 (Debian 15.13-1.pgdg120+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit
```

Citus version:
```
Citus 13.1.0 on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit
```

Settings:

```
|name |setting|unit|
|---------------------------------------|-------|----|
|autovacuum_work_mem |1048576|kB |
|citus.background_task_queue_interval |5000 |ms |
|citus.defer_shard_delete_interval |15000 |ms |
|citus.max_cached_connection_lifetime |600000 |ms |
|citus.max_intermediate_result_size |-1 |kB |
|citus.max_matview_size_to_auto_recreate|1024 |MB |
|citus.node_connection_timeout |30000 |ms |
|citus.recover_2pc_interval |60000 |ms |
|citus.remote_task_check_interval |10 |ms |
|effective_cache_size |9437184|8kB |
|logical_decoding_work_mem |65536 |kB |
|maintenance_work_mem |2097152|kB |
|min_dynamic_shared_memory |0 |MB |
|shared_buffers |1048576|8kB |
|shared_memory_size |8543 |MB |
|work_mem |524288 |kB |
```

Cluster topology:

```
|nodeid |groupid |nodename |nodeport |noderack|hasmetadata|isactive|noderole|nodecluster|metadatasynced|shouldhaveshards|
|----------|----------|-------------|--------------|--------|-----------|--------|--------|-----------|--------------|----------------|
|1 |0 |coordinator |5,432.00000000|default |true |true |primary |default |true |false |
|2 |1 |worker-1 |5,432.00000000|default |true |true |primary |default |true |true |
|3 |2 |worker-2 |5,432.00000000|default |true |true |primary |default |true |true |
|4 |3 |worker-3 |5,432.00000000|default |true |true |primary |default |true |true |
|5 |4 |worker-4 |5,432.00000000|default |true |true |primary |default |true |true |
```

Citus cluster is running inside bare metal Kubernetes. Coordinator's memory requests/limits are 128/136 GB respectively. Kubernetes node that runs coordinator has 1 TB of physical RAM.

Database size is about 80 TB, distributed to 16 shards, almost all of the tables are collocated. The tables are themselves partitioned, so there's about 120000 shards overall.