[avalanche] Fix conflict time initializer to zero

Summary:
std::chrono::duration's default constructor does not initialize to zero. See rationale by the author here: https://groups.google.com/a/isocpp.org/g/std-discussion/c/OcGX7Yj3meI

We saw this break during live testing. A followup patch will include a linter so this issue does not occur in the future.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11759