Protect BigStorage operations with a mutex

[ralimi]

Implement a mutex around BigStorage operations to prevent cleaning up chunks that may be referenced.

This is now technically possible since Delete() is no longer an atomic operation; it deletes keys, then cleans up dangling chunks. However, dangling chunks may be newly-referenced if Set() is invoked at the same time as Delete().

This is unlikely given that this is only done in the Options UI, but we should more systematically prevent it.

We can consider the Web Locks API.