[avalanche] Add stake contender cache to avalanche processor

Summary: This adds the cache to the processor and sanity checks adding contenders to the cache with a simple unit test. The cache is not yet used outside of the unit tests, but this is a necessary prerequisite for testing contender promotion at the processor level.

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17024