Add a SATOSHI constant representing an Amount of one satoshi.

Summary: Use it in various places instead of the GetSatoshi method.

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1493