[ecash-lib] Set the default tx version to 2

Summary: The default tx version in the bitcoind node is 2, CashTab uses version 2, so we should default to version 2 in ecash-lib too.

Test Plan: npm run test && BUILD_DIR=../../build npm run integration-tests && npm run build

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16090