Add destruction function to sub manager middleware

[rekmarks]

• adds destroy function to subscription manager middleware to remove event listener memory leak in the extension background
• bump patch version to 4.1.1
• minor touchups (e.g. remove unused variables/parameters)

[kumavis]

looks great. a couple small things:
for subs middleware:
- [ ] throw error if used after destroyed
- [ ] rename destroyMiddleware -> destroySubscriptions

my notes

- destroy fns
  - [ ] subs
    - [x] sub newHeads
    - [x] sub logs
    - [ ] middleware
      - [ ] throw error if used after destroyed?
      - [ ] rename `destroyMiddleware` -> `destroySubscriptions` or just `destroy`
  - [x] filters
    - [x] middleware

[kumavis]

👑