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>

rustfs/Cargo.toml

@@ -56,6 +56,9 @@ rio-v2 = ["rustfs-ecstore/rio-v2"]
 pyroscope = ["rustfs-obs/pyroscope"]
 # Tokio runtime telemetry. Requires `--cfg tokio_unstable`; use `make build-profiling`.
 dial9 = ["rustfs-obs/dial9"]
+# Allocator features
+mimalloc = ["dep:rustfs-mimalloc", "dep:rustfs-mimalloc-sys"]
+jemalloc = ["dep:tikv-jemallocator"]
 hotpath = [
     "hotpath/hotpath",
     "hotpath/tokio",
@@ -336,11 +339,15 @@ opentelemetry = { workspace = true }
 tracing-opentelemetry = { workspace = true }
 # Data structures
 hashbrown = { workspace = true, features = ["serde", "rayon"] }
-rustfs-mimalloc = { workspace = true }
+rustfs-mimalloc = { workspace = true, optional = true }
 
 [target.'cfg(target_os = "linux")'.dependencies]
 libsystemd.workspace = true
 
+[target.'cfg(not(target_os = "windows"))'.dependencies]
+rustfs-mimalloc-sys = { workspace = true, optional = true }
+tikv-jemallocator = { version = "0.6", optional = true }
+
 [dev-dependencies]
 uuid = { workspace = true, features = ["v4", "v5", "fast-rng", "macro-diagnostics"] }
 serial_test = { workspace = true }