Support CLI top - to return thread information

[tzach]

jvm topreturn JVM thread information.
We also need OS level top to display all OS thread
Information can includes:
ID - thread id
PR - thread prio
S - status,
%CPU
%MEM
TIME+
COMMAND - description

This is a side affect of fixing #23

[nyh]

Some notes:

1. thread->id() is a numeric id of the thread. The comment in sched.hh suggests that it is a long int "guaranteed unique over system lifetime" but this is not correct any more. In fact it is an unsigned int (32-bit) which may wrap around, mimicking Linux process ids. The type and documentation of this function needs to change.
2. thread->priority() returns a thread's floating-point priority, where 1 is the default, 2 means the thread gets half the runtime as usual and 0.5 means the thread gets twice the runtime as usual. If for some reason you want the Posix-like exponential priority, you also have the getpriority() function.
3. A thread does have status (_detached_state->st) like running, queued, etc., but we do not have an API to get it. It will be easy to get.
4. For "%CPU" we need to define what it means (some sort of moving average?). One thing we can do is to expose the runtime the scheduler calculates for each thread (something like export_runtime) but this includes things (like the thread's priority) not normally part of %CPU. Or perhaps we need to a second counter, just for %CPU?
5. "%MEM" is meaningless for individual threads (threads share their memory).
6. For "TIME" we have sched::thread::thread_clock (needs to be documented...), and the normal Posix API clock_gettime(CLOCK_THREAD_CPUTIME_ID).
7. The closest thing we have to "COMMAND" is thread->name().