libglusterfs: Provide a way to disable mempool effects

Problems:
 - Mempool comes in the way of debugging mem-leaks/corruptions
   because tools like valgrind can't detect mem-pools.

 - Accessing freed pointers may crash only when the mem-pool
   is completely exhausted.

Solution:
In DEBUG builds set mempool count to 0 irrespective of the
pool size requested. This should help in debugging
memory-leaks/corruptions.

Change-Id: I7c6a2d696e330f12acff57ebdc589cc150e9ab79
BUG: 1100144
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/7835
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>