RFR: Provide non-taxable allocation slack at the beginning of the cycle

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/pacing-slack/webrev.01/

Current pacing scheme has a little deficiency: it starts with zero budget at the beginning of the
cycle. This means the application threads that want to allocate would need to wait for GC threads to
ramp up and replenish the budget, which introduces latencies like these, even if the GC cycle is fast:

 Actual pacing delays histogram:

       From -         To        Count
       1 ms -       2 ms:           4
       2 ms -       4 ms:         144
       4 ms -       8 ms:           8
       8 ms -      16 ms:           5
      16 ms -      32 ms:           1

The way to deal with this: provide the initial non-taxable slack, so that application can allocate
at the beginning of GC cycle. With this patch, we have zero pacing delays with large enough heap
with adaptive/traversal/partial.

Testing: hotspot_gc_shenandoah, targeted benchmarks

Thanks,
-Aleksey