MINOR: pools: mark most static pool configuration variables as read-mostly

MINOR: pools: mark most static pool configuration variables as read-mostly

The mem_poison_byte, mem_fail_rate, using_default_allocator and the
pools list are all only set once at boot time and never changed later,
while they're heavily used at run time. Let's optimize their usage from
all threads by marking them read-mostly so that them reside in a shared
cache line.