Reduce contention on cache hash table mutex

Previously, the mutex was unnecessarily held while destroying cache
entries in the case that `total_bytes_limit == 0`, leading to contention
on the mutex in the case that destruction is expensive (e.g. release a
large memory allocation).

PiperOrigin-RevId: 688801635
Change-Id: I347f5c6312920bb6b4ae4b3396dbd6414654dfaa

tensorstore/internal/cache/cache.cc

@@ -362,6 +362,9 @@ void StrongPtrTraitsCacheEntry::decrement_impl(
           cache->reference_count_.fetch_sub(CacheImpl::kNonEmptyShardIncrement,
                                             std::memory_order_relaxed);
         }
+        // Release lock before invoking entry destructor, as that may be
+        // expensive.
+        lock = {};
         delete entry_impl;
       }
     } else {