vmem: do not reinitialize the pool[0]->chunks_mtx mutex

The pool[0]->chunks_mtx mutex is locked at chunk.c:178
(called from malloc_init_base_pool() at jemalloc.c:362)
before being initialized at chunk.c:415
(called from malloc_init_base_pool() at jemalloc.c:364).

Move a call to pools_shared_data_create() to pool_new()
after a call to chunk_boot() in order to fix this bug.

Ref: pmem/issues#17