This is inspired by the work of
v0l (https://github.com/v0l/nostr-rs-relay/).
A new trait abstracts the storage layer with an async API. Rusqlite
is still used with worker threads, but this allows for Postgresql or
other backends to be used.
There may be bugs, this has not been rigorously tested.
Implements core NIP-26 delegated event functionality. Events can
include a `delegation` tag that provides a signature and restrictions
on which events can be delegated.
Notable points on the implementation so far:
* Schema has been upgraded to include an index and new column.
* Basic rune parsing/evaluation to implement the example event in the
NIP, but no more.
* No special logic for deletion.
* No migration logic for determining delegated authors for
already-stored events.
