[avalanche] Set the quorum minimal stake to a sane default

Summary: It used to be 0 to avoid breaking the tests, and 10B XEC when the -avalanche flag is set. This diff defaults it to 10B independently of the flag.

Test Plan:

ninja check-extended

Reviewers: #bitcoin_abc, PiRK, sdulfari

Reviewed By: #bitcoin_abc, PiRK, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11917