Caching foundation for a dependencyGraph which maps dependency relations.

[ashprice]

This is an initial PR which lays the groundwork for one that will shortly follow. It introduces lazy caching of pending/completed tasks, a UUID index for pending tasks, the construction of a dependency map, and adds in-place mutation of pending tasks. This roughly halves the instruction count for many reports (though the speed gain from this PR is minor to the one that I will make if it is accepted, about 1.3x for many reports. I gave up waiting for the performance tests in ./performance/ to complete, and instead constructed a smaller, 300 task test database.)

Changes

• pending_tasks() and completed_tasks() return const std::vector<Task>& instead of copying. all_tasks() is deliberately not cached, as it can be very large and is only read once per invocation.
• an index of pending task UUIDs, _pending_index is lazily built with pending_tasks(). get() and modify() use this instead of scanning the vectors.
• A DependencyGraph hashmap is introduced and built from the pending cache. (A temporary map is created for all_tasks(), in the event that dependency_scan() must refer to things in all_tasks.) This is not yet consumed by Task in this PR - that will be the PR to follow.
• modify() makes updates in-place within the vector by leveraging the _pending_tasks index. This cache is invalidated in any case where there may be a change of state between the Rust replica and the cached index.
• some minor changes relating to memory management for pending_tasks and completed_tasks.

Endnotes

This PR doesn't necessarily bring much by itself, but it's needed to lay the groundwork for DependencyGraph to be used by getDependencyTasks() and getBlockedTasks(). Adding in in-place modifications is a somewhat opportunistic addition for the PR, as it's not strictly necessary for core functionality, but it does improve modification performance when running big batch modifications.

All tests pass and I am pretty sure this doesn't break anything, but there's a small chance bugs have slipped by me.

This would be my first major contribution to a C++ codebase, and I am not a professional software engineer, so feedback on pretty much everything here is accepted gratefully.