Implement endpoint to get all events since a block height

[shesek]

Similarly to /txs/since/:height, but returning events instead of transactions. Could be called /events/since/:height.

This would make it easier to catch-up with missed events, without having to implement separate code to extract relevant updates out of transactions.

Internally, bwt should create the events based on the transaction data, rather than keeping the events themselves around. This ensures that no events will be missed even if bwt is down while they happen.

[shesek]

Implemented in 9d619f7.

https://github.com/shesek/bwt#catching-up-with-missed-events--re-org-detection