Moves sweep phase out of stoptheworld by adding
background sweeper goroutine and lazy on-demand sweeping.

It turned out to be somewhat trickier than I expected,
because there is no point in time when we know size of live heap
nor consistent number of mallocs and frees.
So everything related to next_gc, mprof, memstats, etc becomes trickier.

At the end of GC next_gc is conservatively set to heap_alloc*GOGC,
which is much larger than real value. But after every sweep
next_gc is decremented by freed*GOGC. So when everything is swept
next_gc becomes what it should be.

For mprof I had to introduce 3-generation scheme (allocs, revent_allocs, prev_allocs),
because by the end of GC we know number of frees for the *previous* GC.

Significant caution is required to not cross yet-unknown real value of next_gc.
This is achieved by 2 means:
1. Whenever I allocate a span from MCentral, I sweep a span in that MCentral.
2. Whenever I allocate N pages from MHeap, I sweep until at least N pages are
returned to heap.
This provides quite strong guarantees that heap does not grow when it should now.

http-1
allocated                    7036         7033      -0.04%
allocs                         60           60      +0.00%
cputime                     51050        46700      -8.52%
gc-pause-one             34060569      1777993     -94.78%
gc-pause-total               2554          133     -94.79%
latency-50                 178448       170926      -4.22%
latency-95                 284350       198294     -30.26%
latency-99                 345191       220652     -36.08%
rss                     101564416    101007360      -0.55%
sys-gc                    6606832      6541296      -0.99%
sys-heap                 88801280     87752704      -1.18%
sys-other                 7334208      7405928      +0.98%
sys-stack                  524288       524288      +0.00%
sys-total               103266608    102224216      -1.01%
time                        50339        46533      -7.56%
virtual-mem             292990976    293728256      +0.25%

garbage-1
allocated                 2983818      2990889      +0.24%
allocs                      62880        62902      +0.03%
cputime                  16480000     16190000      -1.76%
gc-pause-one            828462467    487875135     -41.11%
gc-pause-total            4142312      2439375     -41.11%
rss                    1151709184   1153712128      +0.17%
sys-gc                   66068352     66068352      +0.00%
sys-heap               1039728640   1039728640      +0.00%
sys-other                37776064     40770176      +7.93%
sys-stack                 8781824      8781824      +0.00%
sys-total              1152354880   1155348992      +0.26%
time                     16496998     16199876      -1.80%
virtual-mem            1409564672   1402281984      -0.52%

LGTM=rsc
R=golang-codereviews, sameer, rsc, iant, jeremyjackins, gobot
CC=golang-codereviews, khr
https://golang.org/cl/46430043