Executor decision caching and single-host optimisations

[Shillaker]

These two changes are tightly coupled as they both stem from a refactor of Executor::executeThreads. This function is part of the Executor API (e.g. called in Faasm to execute batches of threads). It was previously doing a lot of untested logic around caching scheduling decisions and updating snapshots, so updating it was difficult.

This change includes a refactor of that function, and adding single-host optimisations, avoiding dirty tracking and snapshot merging for an app executing on a single host.

Changes:

• Add new CACHED topology hint to move decision caching into the Scheduler.
• Mark BatchExecuteRequests with a singlehost flag if executed only on the main host.
• Only perform dirty tracking and snapshot merging if not executing on solely on the main host.