Over-sized plans captured in completed_requests can result in system instability.

Currently there is no cap on the plan size stored in system:completed_requests.  This can lead to spikes in memory consumption when the plan contents of system:completed_requests are queried, for example by the UI's monitor.  In extreme cases the spikes can lead to outages.

We need to limit the size of plans we'll store.  If a plan for such a statement is needed it can be obtained via profiling or EXPLAIN.  This would then be a one-off user invoked action and would consequently have negligible impact on the system.

The limit should be user configurable but capped to around 20 MiB (the KV document size limit) in case in the future we end up persisting the data to KV storage.