More efficient catch-up behaviour, remote config, ...

* use eth_getLogs to fetch logs in bulk when catching up
  after a restart.
* allow loading configuration from URLs
* switch from toml to json for the configuration
* store block_number as integer in the db instead of hex string
* use ethers_core for types
* make webhook timeout configurable
* limit the number of deliveries loaded from the DB in one query

.gitignore

@@ -2,4 +2,4 @@
 
 db.sqlite
 
-blockwatch.toml
+blockwatch.config.json