When revwing #6807 I assumed that `probSetChoice` worked this way. 

I think that the coverage of various configuration options should
generally track what we expect the actual useage to be to focus the
most test coverage on the configurations that are the most prevelent.

This is just a configuration change to default to using the new stack
unless explicitly disabled (e.g. `UseLegacy`) this renames the
configuration value and makes the configuration logic more clear.

The legacy option is good to retain as a fallback if the new stack has
issues operationally, but we should make sure that most of the time
we're using the new stack.

In the transaction load generator, the e2e test harness previously distributed load randomly to hosts, which was a source of test non-determinism. This change distributes the load generation to the different nodes in the set in a round robin fashion, to produce more reliable results, but does not otherwise change the behavior of the test harness.

I realized after my last commit that my change made a following line of code a bit redundant.

(alternatively my last change was redunadnt to the existing code.)

I took this oppertunity to make some minor cleanups and logging changes to the node changes which I hope will make tests a bit more clear.

This is a mostly-automated fixup using Comby (https://comby.dev) to remove 
lexically-obvious redundant error checks. No functional changes are intended.

To reproduce the core change:

    # Collapse redundant error check conditionals
    % comby -in-place 'if err != nil {
       return err
    }
    return nil' 'return err' .go

    # Fold out unnecessary error temporaries
    % comby -in-place ':[spc~^\s*]err :[~:?]= :[any]
       return err' ':[spc]return :[any]' .go

Fixes #6479 and related cases.

Having looked at our network address parsing and connection code, it
really looks like we're not doing anything on top of what the standard
library is doing (both in terms using `net.ParseIP` and also
`net.Dial`,) and I don't think we need to run the tests 2x the number
of times just to run through different areas of the standard
library. I think most of our users are going to be using IPv4, and
would be down to fully remove this dimension as well, if we find it's
making noise, but for now I think it's fine.

This tweaks sleeps around pertubations, based on a theory that our
tests with "kill" pertubations restart the nodes fast enough the peers
haven't marked it down when it tries to reconnect. In my local test
runs, this clears out *most* of the test failures that I've seen,
except for one evidence-related test-harness problem (which should be
handled separately.)

## Description

Expose p2p functions for use in the sdk. 

These functions could also be copied over to the sdk. I dont have a preference of which is better.

This PR make some tweaks to backfill after running e2e tests:
- Separates sync and backfill as two distinct processes that the node calls. The reason is because if sync fails then the node should fail but if backfill fails it is still possible to proceed.
- Removes peers who don't have the block at a height from the local peer list. As the process goes backwards if a node doesn't have a block at a height they're likely pruning blocks and thus they won't have any prior ones either. 
- Sleep when we've run out of peers, then try again.

## Description

Update for logging changes

Addresses a beginning component of #6255

I believe that this, in my testing seems to help the e2e state-sync
tests complete more reliably, by fixing some potential, range-related
slice building, as well as the way the test app hashes snapshots.

Additionally, and I'm not sure if we want to do this, but I added this
hook to the reactor that re-sends the request for snapshots during the
retry. This helps in tests prevent systems from getting stuck, but I
think in reality, it might create more traffic, and operators would
just restart a state-syncing node to get a similar effect.