Cardano Updates

* I removed an outdated comment
* I renamed datum to datum_option (since it can be a hash or a datum)

Other small changes include:
* Added a trace event `TDBInitialisingFromLMDBDone`.
* Initialisation from an existing LMDB database does not rely on the
default `LMDBLimits` anymore, and is passed a limits argument instead.
TODO: We should decide whether we want to hardcode these limits to a
a substantially large one, or possibly we could link these limits to a
versioning number?

add cwbtc

attempt implementation

Other small changes include:
* Added a trace event `TDBInitialisingFromLMDBDone`.
* Initialisation from an existing LMDB database does not rely on the
default `LMDBLimits` anymore, and is passed a limits argument instead.
TODO: We should decide whether we want to hardcode these limits to a
a substantially large one, or possibly we could link these limits to a
versioning number?

Updated crypto exchange rates chapter. I tried my best to address @rdlrt concerns. I would like to add that English is just my 3rd language, so I hope it is not too bad.

Previously, the sequence number of the database is written to disk as
part of the on-disk database settings on every flush/write. Conceptually
however, the settings of a database should not change on every flush or
write. Instead, we make a dinstinction between on-disk database
"settings" and "state", where the state can be updated in every
write/flush, but the settings should generally be left untouched after
database initialisation or node start-up.

  Fixes #217.

  This works because (a) JavaScript is ultimately single-threaded, (b) there's no execution preemption happening between a 'send' and a 'wait'.