RFC: Dynamic groups support

[Aleksey Shipilev]

Hi,

Following up to Nitsan's (external) and Dmitry's (internal) thread on
asymmetric benchmark support in JMH, I would like to solicit the review
on this change:
 http://cr.openjdk.java.net/~shade/jmh/jmh-dynamic-groups-1.patch

I would like Nitsan and Dmitry to apply this patch over the fresh tip,
and try to build the benchmarks they like. If you see something is
missing there which does not fit your use cases, speak up. I'll do some
polishing meanwhile.

Gruesome details:

 *) This change eliminates the static group assignment, and pushes for
dynamic group assignment. That is, you don't need to set @Threads
anymore for @Group in advance. Instead, you can select the thread group
distribution at runtime.

 There are two ways to control it:
    a) With the new command line option "-tg INT,INT,...,INT".
    b) With the new API method "threadGroups(int... groups)".

 This opens up the way to have a single asymmetric benchmark, and juggle
the thread distribution in runtime.

 *) @Threads is illegal on @Group now, for two reasons: first, it
clashes with the JMH meaning of "hinting the default thread count",
second, the group API is slightly changed, and so we need to break it in
some way. With this change, you will have to use @GroupThreads for
telling JMH how many threads should be the part of group. This opens up
the way to say...

  @Threads(12)
  class T {
     @GMB
     @Group("Z")
     @GroupThreads(1)
     void test1() { ... }

     @GMB
     @Group("Z")
     @GroupThreads(5)
     void test2() { ... }
  }

 This will run 2 groups with {1,5} threads each. Like @Threads,
@GroupThreads is only the hint for the default distribution. Overriding
it from cli/api can produce another distribution.  Notice that by
playing with both @Threads/@GroupThreads, -t/-tg,
.threads()/threadGroups(), you may configure the run quite flexibly.

 *) The number of worker threads now gets automatically rounded to the
group unit. In the example above, running it with -t 13 will
automatically populate 3 groups with {1,5} threads each, totalling 18
threads.

Thanks,
-Aleksey.