BENCHMARKING: Record timing and memory stats for the various steps in futures

[HenrikBengtsson]

Record timing and memory stats for the various steps in futures, e.g. creation, identification of global variables, exporting globals, launching future, time for future to complete, collection of value/exceptions etc. This should be done (optional?) for all types of futures.

[HenrikBengtsson]

For memory profile, we have utils::Rprofmem() which logs all memory allocations done by allocateVector() (part of the native R API). Note that the information logged by Rprofmem() could be improved, but that would require updates to core R, cf. HenrikBengtsson/Wishlist-for-R#25

[HenrikBengtsson]

See also snow::snow.time().