Extend Accumulate...() with a predictive meas. vector resize

[xparq]

...so that adding new measurement items during the sampling phase won't actually trigger reallocation (in most cases).

• The recommended looping pattern of IPROF_SYNC calls lets the first loop cycle "break new ground" and record how many measurements have been made, then (after the actual syncing), that std::swap(measurements, unfinished); could be extended with a preallocation step for the new measurements instance.

• Evaluate if that unfinished vector, recreated on every call, could just be thread_local instead!