This exposes two new asset stats fields: `reissuance_tokens` and `burned_reissuance_tokens`
Also, the asset id of the reissuance token is now available under the `reissuance_token` field of the asset.
Instead of fetching it from the bitcoind rpc. This speeds up startup times significantly.
This required keeping the last indexed block hash in the db, under `t` (for "tip").
The first start-up without the saved tip will still be slow, but once that gets saved,
followup start-ups will become faster.
The presence of the `t` in the db is now also used to determine whether the initial
sync was completed, replacing the `i` flag introduced in the parent commit.
Resolves https://github.com/Blockstream/esplora/issues/76
