database: Wrap EdgeQLConnection objects

[isidentical]

Since we separated EdgeDB-bound logic from the compiler/serializer, maybe we can do the same thing to the database connections.

• Introducing reiz.database package with
  • get_connection(backend: str) -> BaseConnection, connection: BaseConnection = get_connection(config.ir.backend)
  • reiz.database.base a BaseConnection class, which would host 2 main methods (maybe more) get_blocking_connection(), get_async_connection. It might host a pool inside maybe
  • reiz.database.edgeql that implements these methods