RFC v3: memtank, first draft

RFC v3: memtank, first draft

For analogy with mempool, named this structure memtank.
Same a s mempool it allows to alloc/free objects of fixed size
in a lightweight manner (not as lightweight as mempool,
but hopefully close enough).
The whole idea is that alloc/free is used at fast-path and don't
allocate/free more than *min_free* objects at one call.
So for majority of cases our fast-path alloc/free should use
rte_ring enqueue/dequeue calls.
Also user will need to call grow/shrink periodically
(ideally from the slow-path) to make sure there is enough
free objects in the tank.
Internally it is just a simple LIFO for up to *max_free* objects plus
a list of memory buffers (memchunk) from where these objects were
allocated.

v1 -> v2
 - Added UT
 - Fixed few bugs

v2 -> v3
 - extend UT with more parameters

Change-Id: Ibb993453ce67391bb86ad3f3c9bb192f07d7bb35
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>