[ecash-wallet] Send tokens to an arbitrary number of recipients in a single action with chained token txs

Summary:
Add support for sending a token to many outputs. Abstract away token spec and OP_RETURN limit considerations to support an interface that accepts action-focused results, i.e. what addresses get what quantity of token.

The feature makes token-to-token airdrops trivial.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19316