Remove the RwLock from the HyperlaneMessages db

[damiannolan]

Description

The messages db currently contains the following:

pub struct HyperlaneMessageStore {
    pub db: Arc<RwLock<DB>>,
}

Update this to remove the RwLock around the DB.
The rocksdb DB is already Send + Sync, internally synchronized, and designed for concurrent use.

Likewise the same should be done for the snapshot store if it is being kept but I believe the intention is to remove it completely.