vmem: add the malloc_rwlock_t type
vmem: change type of pool->arenas_lock to malloc_rwlock_t
vmem: fix race condition in je_mallocx()

Ref: pmem/issues#15
Merge pull request #79 from ldorau/vmem-fix-sporadic-segfault-in-MALLOCX_ARENA-ut

vmem: fix sporadic segfault in MALLOCX_ARENA unit test