Add new mode to apply load for tx set validation

[SirTyson]

Description

This adds a new mode to apply load, allowing us for more "end to end" simulations. In the new mode, we time and simulate ballot phase through the end of apply, but in a single node network such that SCP state transitions occur instantly. This is useful, as it allows us to benchmark the processing overhead associated with consensus, as well as measure cache performance and dedup similar work done during tx set validation and the later application of this transaction set.

The motivation for this test was from my recent block latency experiments. I noticed how optimizations in network calls, reducing bandwidth, etc were not moving the needle on actual block latency. It turned out that consensus latency was greatly affected by CPU based bottlenecks, not just network calls. On supercluster, it's challenging to benchmark CPU costs of concensus, as you can't easily run tracy or profilers and have limited visibility to single node performance. This test allows us to much more easily measure and improve the processing heavy aspects of concensus. In overlay-v2-shared for a block with 6K SAC transfers, we saw about 300 ms of non-network "ingestion" time when receiving a tx set and 500 ms of apply. Imo this was a significant blind spot in our single node apply load tests previously.

Note that the actual number produced by this test is meaningless, as consensus without a network tells us very little about overall performance. However, the reported phase timings are very useful in identifying bottlenecks and comparing solutions. This simulates the processing done by a non-leader node. In the future, it may also be helpful to add a mode for block construction as well.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[Copilot]

Pull request overview

Adds an optional apply-load benchmark path covering tx-set decoding, validation, local consensus, and ledger application.

Changes:

• Adds configurable validation-and-apply timing.
• Records phase timings and signature-cache metrics.
• Documents the mode and new validation metric.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/simulation/ApplyLoad.h	Declares timing paths and phase statistics.
src/simulation/ApplyLoad.cpp	Implements consensus-backed benchmarking.
src/main/Config.h	Defines timing configuration.
src/main/Config.cpp	Parses timing configuration.
src/herder/HerderSCPDriver.h	Declares validation timer.
src/herder/HerderSCPDriver.cpp	Instruments tx-set validation.
docs/software/commands.md	Documents apply-load timing paths.
docs/metrics.md	Documents the validation metric.
docs/apply-load-benchmark-sac.cfg	Adds example timing configuration.

[SirTyson]

Should have addressed all the comments, sorry for the delay, I've been in bug finding mode...

[dmkozh]

LGTM, but I'm not sure if master is open yet.