Add RngUtils::sample() method for reservoir sampling from iterators
an attempt at a singleton pausible idle callback for each scheduler. suffers from nondeterministic deadlock and also pending scheduler messages on scheduler shutdown.
std::rt: Touch up idle logic
std::rt: Fix a race in the UvRemoteCallback dtor
std::rt: Fix a race in UvRemoteCallback's dtor that misses callbacks

Full description in comments.