notmuch: remove Context in nm_read_entire_thread()

notmuch: remove Context in nm_read_entire_thread()

Removed the coupling between `nm_read_entire_thread()` and `struct
Context` by replacing it with `struct Mailbox`.

`Context` had two uses: `Mailbox` reference source and `ctx_update()`.

The introduction of mailbox notification code allows mailbox events to
trigger `ctx_update()`, which removes the hard dependency on `Context`.

Passing a `Mailbox` reference removes the remaining soft dependency on
`Context`.