[ecash-lib] Add txid computation from the Tx object

Summary:
In order to efficiently use the txid subscription from chronik we need to compute this txid. Doing so makes it possible to get the tx added to mempool event from chronik after it is broadcast.

Depends on D18393.

Test Plan:

npm test

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18394