Safe DB connection sharing

[richardhuaaa]

This refactor aims to do the following:

1. Continue allow sharing DB connections between libxmtp logic and the OpenMLS keystore, including via transactions
2. Prevent errors arising from concurrent mutable access to this connection (whether at compile-time via multiple &mut refs or run-time via multiple RefCell borrows)
3. Simplify the DB interface

This is done by:

1. Create an DbConnection struct that wraps the RawDbConnection in a RefCell. The DbConnection uses interior mutability, therefore a non-mut reference to it can be freely shared as many times as desired.
2. Move all DB operations, as well as fetch and store onto this struct. Additionally, add a method to this struct that allows external callers to execute raw queries via a closure. This ensures that the RawDbConnection is only ever accessed internally within DbConnection, and uses function scope to make sure that borrows on the RefCell are always returned before they are used again.
3. Use visibility to ensure that the RawDbConnection is completely inaccessible outside of EncryptedMessageStore and DbConnection. It is only possible for external callers to interact with the database via DbConnection, and all references to RawDbConnection in the code have been replaced by DbConnection.

[richardhuaaa]

This is a beefy refactor - feel free to directly modify/merge if you are blocked on this/want to avoid conflicts

[insipx]

Cleaner and so much better, nice job!

[neekolas]

These changes look great. Think my only beef is with the name. Not sure slapping Xmtp onto the front of the name adds any useful context. Given this is the only DbConnection anyone is using, I'd advocate for just calling it something straightforward.