[avalanche] Rename the orphan pool to immature pool

Summary:
And rename the related constants as well. The pool no longer contains orphans but proofs with immature utxos. The comments and variables will be updated in another diff.

Depends on D11861.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11864