epoch: Fix false sharing in Local struct (crossbeam-rs#1026)

The perf c2c data shows lots of cacheline contention were caused by
false sharing between epoch and other field in "Local" struct. So
use CachePadded class to align it to another cacheline to resolve
this issue.

crossbeam-epoch/src/internal.rs

@@ -292,9 +292,6 @@ pub(crate) struct Local {
     /// A node in the intrusive linked list of `Local`s.
     entry: Entry,
 
-    /// The local epoch.
-    epoch: AtomicEpoch,
-
     /// A reference to the global data.
     ///
     /// When all guards and handles get dropped, this reference is destroyed.
@@ -315,6 +312,9 @@ pub(crate) struct Local {
     prev_epoch: Cell<Epoch>,
 
     collecting: Cell<bool>,
+
+    /// The local epoch.
+    epoch: CachePadded<AtomicEpoch>,
 }
 
 // Make sure `Local` is less than or equal to 2048 bytes.
@@ -343,7 +343,6 @@ impl Local {
 
             let local = Owned::new(Local {
                 entry: Entry::default(),
-                epoch: AtomicEpoch::new(Epoch::starting()),
                 collector: UnsafeCell::new(ManuallyDrop::new(collector.clone())),
                 bag: UnsafeCell::new(Bag::new()),
                 guard_count: Cell::new(0),
@@ -352,6 +351,7 @@ impl Local {
                 advance_count: Cell::new(0),
                 prev_epoch: Cell::new(Epoch::starting()),
                 collecting: Cell::new(false),
+                epoch: CachePadded::new(AtomicEpoch::new(Epoch::starting())),
             })
             .into_shared(unprotected());
             collector.global.locals.insert(local, unprotected());