This regression is still present in 9.1 GA as the cherrypick of revert
commit was missed during release.

Re-applies #3544 (reverted in #3756 due to ~20% SET regression) with the
performance fix from #3760.

**Root Cause:** The original #3544 changed
`tryOffloadFreeObjToIOThreads` to only offload the SDS buffer free to IO
threads, freeing the `robj` shell on the main thread. I carried out
profiling for the change and it showed that freeing the `robj` shell on
the main thread became the prime main-thread hotspot (~10% CPU), while
IO threads shifted from doing real `jemalloc` work to spinning idle on
`spmcDequeue`.

**Fix**: Keep `tryOffloadFreeObjToIOThreads` offloading the entire robj
(`decrRefCount`) to the IO thread. Cross-thread `zfree` is safe with
`jemalloc`.

This PR includes all cleanup work from #3544 so - 
- `trySendWriteToIOThreads`: defer clearing `last_header` until after
successful enqueue
- `evictClients`: simplified bookkeeping
- Queue sizes as runtime parameters instead of compile-time macros
- IO ignition policy using `stat_active_time` instead of `getrusage`
- Function renames (`IOThreadFreeArgv` --> `ioThreadFreeArgv`, etc.) and
doc comments

**Benchmark** on (Graviton4 c8gb.metal-48xl):
Config: SET, 128B values, 9 IO threads, pipeline=10, 1600 clients - Same
as Valkey official method
| Version | Throughput |
|---------|-----------|
| Unstable + original #3544 | ~1,554K rps |
| Unstable + this PR | ~2,116K rps |

<details>
<summary>Diff vs original #3544 (perf fix)</summary>

```diff
diff --git a/src/io_threads.c b/src/io_threads.c
--- a/src/io_threads.c
+++ b/src/io_threads.c
@@ // IO thread handler
 case JOB_REQ_FREE_OBJ:
-    zfree(data);
+    decrRefCount(data);
     break;

@@ // tryOffloadFreeObjToIOThreads
-    /* We offload only the free of the ptr that may be allocated by the I/O thread.
-     * The object itself was allocated by the main thread and will be freed by the main thread. */
-    void *job = tagJob(sdsAllocPtr(objectGetVal(obj)), JOB_REQ_FREE_OBJ);
+    void *job = tagJob(obj, JOB_REQ_FREE_OBJ);
     if (unlikely(spmcEnqueue(&io_shared_inbox, job) == false)) return C_ERR;
-    objectSetVal(obj, NULL);
-    decrRefCount(obj);
     io_jobs_submitted++;
```
</details>

---------

Signed-off-by: Roshan Khatri <rvkhatri@amazon.com>