ts: multiple improvements mostly to timers

Multiple opportunities for performance gains where found improving ts-standalone.

• Timers are now implemented as Futures for Oneshots and Streams for Intervals. This is more consistent with how they supposed to be used and it keeps Oneshots simpler.
• "After" timers (the one that fire no sooner than the target instant) now use their own data structure in the reactor. We no longer need to use the wait / 2 hack. They also use their own types & implementations.
• Fixed Intervals which could fire several times in a row before the interval had elapsed.
• ts-standalone push strategy and statistics were improved. This helped identify the incorrect behaviour for intervals. Fixing those required improving the statistics produced by the tool.
• Other commits mostly improve CPU and memory usage a bit. See commit messages.