fix(mempool): inflight notes are considered unauthenticated once pruned

[Mirko-von-Leipzig]

This PR fixes a panic in the mempool which can occur when under extreme load.

The root cause is some missing book keeping around block pruning. What happens is that

1. transaction A creates note A'
2. transaction B consumed note A', this succeeds because the mempool knows the note is inflight.
3. transaction A is committed in block C
4. block C is eventually pruned, removing note A' from all graphs
5. transaction B is then selected for inclusion in a batch
6. batch DAG panic! because B's unauthenticated note A' is now unknown

The fix is to mark all notes from pruned blocks as authenticated (since we know they definitely exist and are committed). The fix was tested and confirmed using TDD.

This only occurred under extreme load because it required enough transactions to delay selection of B to after A was both committed and pruned.

Fixes #2079

[kkovaacs]

LGTM!