Merge bitcoin/bitcoin#26933: mempool: disallow txns under min relay fee, even in packages

bf77fc9cb45209b9c560208c65abc94209cd7919 [test] mempool full in package accept (glozow)
b51ebccc28e66c1822ab22d2d178be55c6618196 [validation] set PackageValidationState when mempool full (glozow)
563a2ee4f564c8ea5f8313d711b196e260568c04 [policy] disallow transactions under min relay fee, even in packages (glozow)
c4554fe894d7af8e666f5d424deccddf516713ef [test] package cpfp bumps parents <mempoolminfee but >=minrelaytxfee (glozow)
ac463e87df728689701810c3961155c49fdc5b31 [test util] mock mempool minimum feerate (glozow)

Pull request description:

  Part of package relay, see #27463.

  Note that this still allows packages to bump transactions that are below the dynamic mempool minimum feerate, which means this still solves the "mempool is congested and my presigned 1sat/vB tx is screwed" problem for all transactions.

  On master, the package policy (only accessible through regtest-only RPC submitpackage) allows 0-fee (or otherwise below min relay feerate) transactions if they are bumped by a child. However, with default package limits, we don't yet have a DoS-resistant way of ensuring these transactions remain bumped throughout their time in the mempool. Primarily, the fee-bumping child may later be replaced by another transaction that doesn't bump the parent(s). The parent(s) could potentially stay bumped by other transactions, but not enough to ever be selected by the `BlockAssembler` (due to `blockmintxfee`).

  For example, (tested [here](https://github.com/glozow/bitcoin/commits/26933-motivation)):
  - The mempool accepts 24 below-minrelayfeerate transactions ("0-fee parents"), all bumped by a single high-fee transaction ("the fee-bumping child"). The fee-bumping child also spends a confirmed UTXO.
  - Two additional children are added to each 0-fee parent. These children each pay a feerate slightly above the minimum relay feerate (e.g. 1.9sat/vB) such that, for each 0-fee parent, the total fees of its two children divided by the total size of the children and parent is above the minimum relay feerate.
  - If a block template is built now, all transactions would be selected.
  - A transaction replaces the the fee-bumping child, spending only the confirmed UTXO and not any of the outputs from the 0-fee parents.
   - The 0-fee parents now each have 2 children. Their descendant feerates are above minrelayfeerate, which means that they remain in the mempool, even if the mempool evicts all below-minrelayfeerate packages.
   - If a block template is built now, none of the 0-fee parents or their children would be selected.
   - Even more low-feerate descendants can be added to these below-minrelayfeerate packages and they will not be evicted until they expire or the mempool reaches capacity.

  Unless we have a DoS-resistant way of ensuring package CPFP-bumped transactions are always bumped, allowing package CPFP to bump below-minrelayfeerate transactions can result in these problematic situations. See #27018 which proposes a partial solution with some limitations, and contains discussion about potential improvements to eviction strategy. While no adequate solution exists, for now, avoid these situations by requiring all transactions to meet min relay feerate.

ACKs for top commit:
  ajtowns:
    reACK bf77fc9cb45209b9c560208c65abc94209cd7919
  instagibbs:
    re-ACK https://github.com/bitcoin/bitcoin/pull/26933/commits/bf77fc9cb45209b9c560208c65abc94209cd7919

Tree-SHA512: 28940f41493a9e280b010284316fb8caf1ed7b2090ba9a4ef8a3b2eafc5933601074b142f4f7d4e3c6c4cce99d3146f5c8e1393d9406c6f2070dd41c817985c9

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