acl-plugin: avoid crash in multithreaded setup adding/deleting ACLs with traffic...

acl-plugin: avoid crash in multithreaded setup adding/deleting ACLs with traffic (VPP-910/VPP-929)

The commit fixing the VPP-910 and separating the memory operations
into separate heaps has missed setting the MHEAP_FLAG_THREAD_SAFE,
which quite obviously caused the issues in the multithread setup.
Fix that.

Also, add the debug CLIs
"set acl-plugin heap {main|hash} {validate|trace} {1|0}"
to toggle the memory instrumentation, in case we ever need it
in the future.

Change-Id: I8bd4f7978613f5ea75a030cfb90674dac34ae7bf
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>