Make sync-server storage methods async

[dbr]

As discussed in #206

Currently the sync-server StorageTxn trait is all sync, which will mean the HTTP API will be blocked by any database IO

Since async trait methods aren't implemented in Rust (and aren't likely to be soon - based on rust-lang/rust#62149 "Async trait methods" they are dependent on rust-lang/rust#44265 which is still in progress), simplest solution is to use this crate:

https://github.com/dtolnay/async-trait

This would probably tie in with creating a more high-performance storage backend option like Postgres