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.
NIP-01 prefix search, and NIP-12 generic tags are no longer marked as
experimental.
NIP-11 relay info advertises NIP-09 event deletion and NIP-12 generic
tag search support.
The `reject_future_limits` option can now be disabled, and is by
default.
NIP-11 advertises support for created_at limits.
The message for future-dated events has been modified, to be closer to
the recommended example in the NIP.
