Annotate PoolMalloc memory in valgrind builds (#1946)

Annotate PoolMalloc memory in valgrind builds (#1946)

MemPoolMalloc code (i.e. memory_pools code used by default) was missing
VALGRIND_MAKE_MEM_*() calls. Similar calls do exist in MemPoolChunked
code (i.e. code enabled by setting MEMPOOLS environment variable to 1).

Even with these markings, "memory_pools on" configuration is still not
quite compatible with Valgrind leak reporting suppressions: In some
cases, Valgrind may still incorrectly suppress leak reporting (or report
leaks that should have been suppressed) because Valgrind associates leak
suppressions with memory _allocators_ while buggy code may leak memory
allocated by others. The long-term solution (if it exists) requires
upgrading these markings to VALGRIND_MEMPOOL_*() API targeting memory
pools, but that requires a serious effort, especially when dealing with
MemPoolChunked complexities. The added markings help detect non-leak
violations and improve PoolMalloc/MemPoolChunked code symmetry.