Use FIFO for the bootstrap cache

[RolandSherwin]

• Simplify the bootstrap cache to be a FIFO.
• Since there is a change in struct, we've also introduced some backwards compatibility such that we are able to write the cache in the old format.
  • This is required mostly by our public bootstrap cache, which can still be used by nodes running the older version of the code.
• Introduce --write-older-cache-file argument for antnode and antctl. This also writes the argument inside the antctl's NodeRegistry to preserve the arg during upgrades.
• Add more tests for the ant bootstrap crate.