Fix memory leak.

A missing destructor caused a memory leak in the UDS backend for PDX.
Add the destructor and use std namespace for realloc/malloc/free.

Bug: 63026993
Test: Ran pdx_benchmarks for 400M iterations before and after fix.
      Confirmed that heap grows before fix and is stable after fix.
      Confirmed that VR service reported in the bug now have stable
      memory use.

Change-Id: If870a5db50dfaf5f212285824a1803f34acdabc7