Clean up per-thread mheap setup.

Clean up per-thread mheap setup.

The stats thread was sharing the main mheap when we started at least
one worker or I/O thread, but ran on its own mheap when we started 0
worker + io threads.

Net of this change; if a VLIB_REGISTER_THREAD instance specifies a
per-thread mheap, a per-thread mheap will be provided. Otherwise,
threads share the main heap.

The stats thread now uses the main heap. Simpler is better.

Change-Id: I1fff0dd66ae8f7dfe44923f702734e2832b55b09
Signed-off-by: Dave Barach <[email protected]>