Fix deadlock

[coolreader18]

Description of Changes

Call order that would lead to deadlock:

module host: read-lock subscriptions
module host: commit tx, release lock on db
add_subscriber: take read lock on db
module host: call DatabaseUpdate::from_writes. tries to take write lock on db. cannot, add_subscriber has a read lock
add_subscriber: tries to take write lock on subscriptions. cannot, module host still holds it
deadlock

This fixes by making from_writes not take a write-lock on the db

[coolreader18]

Not sure if this is okay - the mut version does a db scan but this seems simpler but maybe it can be out of sync with the system table? idk, but this is fine for now.

[jdetter]

I've tested this and it does fix the deadlock, there does seem to be a correctness issue present in master but I think it probably was not introduced by this PR. On that basis I'm fine to approve 👍