tests: add test for free() throughput across CPUs

There is one allocating threads and two freeing threads. Each thread
is allocated on a different core. The test measures throughput of
objects freed by both threads.
mempool: remove contention inside free_different_cpu()

Fixes #308.

When the per-cpu-pair ring fills up, the freeing thread is blocked and
enters a synchronous object hand-off. That synchronous hand-off is
cause of contention. Instead of having a bounded ring we can use an
unordered_queue_mpsc which links the freed objects in a chain. In this
implementation push() always succeeds and we don't need to block.

In a test which allocates 1K blocks on once CPU and having two threads
freeing them on two other CPUs, there is a ~40% improvement of free()
throughput.

I tested various implementaions, based on different queues. Statistics
of free/sec reported by misc-free-perf (one sample = one run):

current:

  avg =  8133055.09
  stdev = 118322.06
  samples = 5

ring_spsc<1M> (no blocking):

  avg = 10442665.98
  stdev = 476334.93
  samples = 5

unordered_queue_spsc:

  avg = 10258212.69
  stdev = 418194.22
  samples = 5

unordered_queue_mpsc:

  avg = 11701334.99
  stdev = 725299.97
  samples = 5

Testing showed that unordered_queue_mpsc() performs best in this case.

Dead objects are collected by per-CPU worker thread (same as
before). The thread is woken up once every 256 frees. That threshold
was chosen so that the behavior would more or less correspond to what
was before.
sched: turn _migration_lock_counter into __thread local variable

This way migration-lock can be used unconditionally in memory
allocator which is called very early, before the first thread is
created.
mempool: prevent across-CPU page stealing

This addresses the race which is mentioned in the comment. If the
thread doing add_page() is migrated before the preempt_lock is taken
it may feed the pool with a page coming from different CPU's page
buffer.

The fix is to embrace the body with migration-lock.
mempool: do not take preempt_lock when freeing from different cpu
mempool: remove contention inside unfill_page_buffer()

This change reduces page buffer pressure caused by virtio-net
allocations, showign up as contention inside unfill_page_buffer() and
refill_page_buffer(). The virtio-net subsystem allocates pages on one
CPU which are then scattered across all other CPUs and freed to their
per-CPU buffers. To mitigate that, this change enables caching of
pages used by virtio-net.
trace: support time slicing options in `trace summary`

The summary will only include samples within the specified range.
Timed samples will be trimmed to the given time range.

Requested-by: Glauber Costa <glommer@cloudius-systems.com>