perf(ecstore): add bucket existence cache and allocator feature flags (#6496)

## Bucket existence cache
- Add BucketExistenceCache in crates/ecstore/src/disk/fs.rs
- Cache bucket directory existence checks with 60s TTL
- Replace access() calls with cached_access() in local.rs
- Add invalidate_bucket_cache() for cache invalidation on create/delete
- Reduces statx syscalls by 89% (from 10,716/s to 1,186/s)

## Allocator feature flags
- Add mimalloc and jemalloc features to rustfs/Cargo.toml
- Default: system allocator (Rust built-in)
- --features mimalloc: mimalloc allocator
- --features jemalloc: jemalloc allocator
- Allows A/B testing different allocators

## Performance impact
- 1KiB PUT: 861 obj/s (unchanged, futex is main bottleneck)
- statx reduction: 89% (from 10,716/s to 1,186/s)
- Main bottleneck remains mimalloc internal synchronization

Ref: rustfs/backlog#2005
Ref: microsoft/mimalloc#1372

Co-authored-by: hector <hetor@rustfs.com>
Co-authored-by: heihutu <heihutu@gmail.com>
Co-authored-by: overtrue <anzhengchao@gmail.com>

Cargo.lock

@@ -9273,6 +9273,7 @@ dependencies = [
  "temp-env",
  "tempfile",
  "thiserror 2.0.20",
+ "tikv-jemallocator",
  "time",
  "tokio",
  "tokio-rustls",
@@ -11998,6 +11999,26 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "tikv-jemalloc-sys"
+version = "0.6.1+5.3.0-1-ge13ca993e8ccb9ba9847cc330696e02839f328f7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cd8aa5b2ab86a2cefa406d889139c162cbb230092f7d1d7cbc1716405d852a3b"
+dependencies = [
+ "cc",
+ "libc",
+]
+
+[[package]]
+name = "tikv-jemallocator"
+version = "0.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0359b4327f954e0567e69fb191cf1436617748813819c94b8cd4a431422d053a"
+dependencies = [
+ "libc",
+ "tikv-jemalloc-sys",
+]
+
 [[package]]
 name = "time"
 version = "0.3.55"