While profiling command execution, I noticed that command argv object
alloc/free overhead is quite high for workloads with many small
arguments (e.g. `HSET` with many fields). The effect is much more
visible with pipelining when Redis becomes CPU bound.

I experimented with replacing argv object alloc/free with a simple
object pool and saw significant speedups.
(Note: related effort around this topic:
#13726)

In this PR, I tried to improve the main hotspots in the memory
allocation path (focusing on command arg allocations) to close the gap
with custom pool performance, so we can avoid having a dedicated memory
pools and let the whole codebase benefit from these optimizations.

## Changes

### 1) Faster dealloc via passing size hint to jemalloc (separate PR
#15071)
Jemalloc does more work than an object pool on free (a lookup on a tree
to find the allocation's size class). For some deallocations, we can
reduce free path overhead by passing a size hint to jemalloc (i.e.
`sdallocx()`) which can skip metadata lookup in the common case. This PR
introduces `zfree_with_size()` and uses it where we can know the
allocation size i.e. `OBJ_ENCODING_EMBSTR` objects in `decrRefCount()`
and SDS free path.

### 2) Reduce atomic operation cost for stat updates
`update_zmalloc_stat_alloc()` / `update_zmalloc_stat_free()` previously
used atomic read-modify-write (RMW) operations (`atomicIncrGet` /
`atomicDecr`) which can emit expensive locked instructions on x86.

When we can guarantee a single writer to a counter, we can use a cheaper
load+add+store sequence instead of a locked RMW. This PR gives the first
16 threads dedicated slots for used_memory stats (intended to cover the
main thread/ I/O threads) so they can use this single writer fast path.
Threads beyond that fall back to a shared pool and continue to use full
atomic RMW.

### 3) Improve jemalloc tcache hit rate 

With the default `lookahead=16` config, a pipelined HSET with ~20 fields
does ~40 small allocations per command (fields + values), so you can get
16 x 40 = ~640 allocations. When args are small, many of these land in
the 32 byte size class (often `EMBSTR`). Jemalloc’s default per-bin
tcache cap is 200, so this kind of burst overflows the cache and it does
frequent flushes. I raised the small-bin tcache limits
(lg_tcache_nslots_mul:3, tcache_nslots_small_max:1000) to handle these
bursts better. In the worst case, tcache may have a higher memory usage
due to this change. Perhaps, another option was lowering `lookahead` to
tune it differently.

### 4) Inlining
When you have a simple pool, it has a few small functions and it is easy
for compiler to inline them. Compared to that, jemalloc alloc/free path
has a deeper call stack. Also, jemalloc was not compiled with `-flto`
which was preventing inlining jemalloc functions. As part of this PR, I
added `-flto` flag to jemalloc when it is enabled for Redis.

Compiler also chooses not to inline some hot path functions in Redis.
This suggests PGO (profile-guided optimization) could provide additional
wins and perhaps we can start experimenting with it sometime. We could
try to force inlining with attributes like `always_inline` but it is
hard to apply across a deep call stack and misuse can cause code bloat.
So, rather than going in this direction, I added `inline` keyword to
some functions for now. This doesn't make compiler to inline all hot
path functions but at least it is a step ahead. (If we can further
improve this in future, performance gets very close to custom memory
pool implementation).

## Benchmark results

Commands were like:

```
memtier_benchmark   --command="HSET __key__ username john_doe email [email protected] password hashed_pwd_123 created_at 1709125200 updated_at 1709125200 first_name John last_name Doe phone_number +1234567890 address 123_Main_St city NewYork country USA postal_code 10001 company Acme_Corp job_title Engineer bio Loves_coding"   --command-ratio=1   --command-key-pattern=P   --key-prefix="hsetkey"   --key-minimum=1   --key-maximum=100000   -n 1000000   -c 50   -t 2   --hide-histogram --pipeline 50
```

| Benchmark | Improvement |
| --- | ---: |
| SET | +0% |
| SET (pipeline) | +8% |
| HSET 15 fields | +2% |
| HSET 15 fields (pipeline) | +17% |
| ZADD 15 elements| +3% |
| ZADD 15 elements (pipeline) | +15% |