vppinfra: don't call dlmalloc API directly from the code

vppinfra: don't call dlmalloc API directly from the code

 - it is confusing from end consumer perspective that some thing
   is somewhere called heap and somewhere mspace

 - this is base for additional work where heap pointer is not the same
   thing like mspace

Type: improvement
Change-Id: I644d5a0de17690d65d164d8cec3c5654571629ef
Signed-off-by: Damjan Marion <damarion@cisco.com>