OPTIM: fd: Make each thread group's FD table NUMA-local

OPTIM: fd: Make each thread group's FD table NUMA-local

With per-thread-group FD tables, the tables are allocated and zeroed,
hence faulted in, by the main thread, and the kernel's first-touch
placement then places every group's table on the main thread's NUMA
node, making the other groups' fdtab accesses, including the atomics
done for every FD event, run on remote memory.

Allocate the tables with mmap() instead, so that their pages are only
faulted in when they are first written, and make these first writes
come from the right CPUs: the boot thread is bound to group 0's CPUs
before any FD is inserted, and to each other group's CPUs around that
group's copy of the boot table, so that every table stays local to its
own node. Each group's slice is padded to the kernel's page size so
that no page is shared between two groups. Only matters with
per-group FD tables.