Relax assumptions about in-mempool children of in-mempool
parents. With package RBF, we will allow a package of size
2 with conflicts on its parent and reconsider the parent
if its fee is insufficient on its own.

Consider:

TxA (in mempool) <- TxB (in mempool)

TxA (in mempool) <- TxB' (in package, conflicts with TxB) <-
TxC (in package)

If TxB' fails to RBF TxB due to insufficient feerate, the
package TxB' + TxC will be considered. PackageV3Checks
called on TxB' will see an in-mempool parent TxA, and
see the in-mempool child TxB. We cannot assume there is
no in-mempool sibling, rather detect it and fail normally.

Prior to package RBF, this would have failed on the first
conflict in package.

Support package RBF where the conflicting package would result
in a mempool cluster of size two, and each of its direct
conflicts are also part of an up-to-size-2 mempool cluster.

This restricted topology allows for exact calculation
of miner scores for each side of the equation, reducing
the surface area for new pins, or incentive-incompatible
replacements.

This allows wallets to create simple CPFP packages
that can fee bump other simple CPFP packages. This,
leveraged with other restrictions such as V3 transactions,
can create pin-resistant applications.

Future package RBF relaxations can be considered when appropriate.

Co-authored-by: glozow <gloriajzhao@gmail.com>
Co-authored-by: Greg Sanders <gsanders87@gmail.com>