Get Thread ID in a portable way

[mratsim]

Currently the threadID is leaking into all datastructures.

It seems like "obvious facilities" do not fit:

• pthread_self return a pthread handle
• gettid is Linux only
• syscall(__NR_gettid) is Linux only (and involves a syscall)
• pthread_getthreadid_np is BSD only
• getThreadID is Windows-only

This is necessary to decouple the memory management and allow upstreaming it.

LookAside list:

weave/weave/memory/lookaside_lists.nim

Lines 49 to 60 in ed177ff

	# Stack pointer
	top: T
	# Mempool doesn't provide the proper free yet
	freeFn*: proc(threadID: int32, t: T) {.nimcall, gcsafe.}
	threadID*: int32 # TODO, memory pool abstraction leaking
	# Adaptative freeing
	count: int
	recentAsk: int
	# "closure" - This points to the proc + env currently registered in the allocator
	# It is nil-ed on destruction of the lookaside list.
	#
	registeredAt: ptr tuple[onHeartbeat: proc(env: pointer) {.nimcall.}, env: pointer]

Mempool free requires the caller to supply its ThreadID

weave/weave/memory/memory_pools.nim

Lines 484 to 519 in bbc677b

	proc recycle*[T](myThreadID: int32, p: ptr T) {.gcsafe.} =
	## Returns a memory block to its memory pool.
	##
	## This is thread-safe, any thread can call it.
	## It must indicates its ID.
	## A fast path is used if it's the ID of the borrowing thread,
	## otherwise a slow path will be used.
	##
	## If the thread owning the pool was exited before this
	## block was returned, the main thread should now
	## have ownership of the related arenas and can deallocate them.
	# TODO: sink ptr T - parsing bug to raise
	# similar to https://github.com/nim-lang/Nim/issues/12091
	preCondition: not p.isNil
	let p = cast[ptr MemBlock](p)
	# Find the owning arena
	let arena = p.getArena()
	if myThreadID == arena.meta.threadID.load(moRelaxed):
	# thread-local free
	if arena.meta.localFree.isNil:
	p.next.store(nil, moRelaxed)
	arena.meta.localFree = p
	else:
	arena.meta.localFree.prepend(p)
	arena.meta.used -= 1
	if unlikely(arena.isUnused()):
	# If an arena is unused, we can try releasing it immediately
	arena.allocator[].considerRelease(arena)
	else:
	# remote arena
	let remoteRecycled = arena.meta.remoteFree.trySend(p)
	postCondition: remoteRecycled

References
https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-getthreadid
https://stackoverflow.com/questions/21091000/how-to-get-thread-id-of-a-pthread-in-linux-c-program