Make Tokenserver DbPool#get async

[ethowitz]

Currently, the DbPool#get method is synchronous, which means that the thread blocks while it waits for a DB connection. We should make this method async, so the thread can pick up other work while waiting for a connection.

Acceptance Criteria

• Use the async_trait crate to make the DbPool#get method async (similar to how it's done for the Syncstorage MySQL DB pool)