Lazily initialise TLS on slow paths.

Copying an idea from mimalloc, initialise the TLS variable to a global
allocator that doesn't own any memory and then lazily check when we hit
a slow path (which we always do when using the global allocator, because
it doesn't own any memory) if we are the global allocator and replace
it.

There is a slight complication compared to mimalloc's version of this
idea.  Snmalloc collects outgoing messages and it's possible for the
first operation in a thread to be a free of memory allocated by a
different thread.  We address this by initialising the queues with a
size value indicating that they are full and then do the lazy check when
about to insert a message that would make a queue full.  This will then
trigger lazy creation of an allocator.

Global initialisation doesn't work for the fake allocator, so skip most
of its constructor.

src/test/perf/contention/contention.cc

@@ -75,7 +75,7 @@ size_t swapcount;
 
 void test_tasks_f(size_t id)
 {
-  Alloc* a = ThreadAlloc::get();
+  Alloc*& a = ThreadAlloc::get();
   xoroshiro::p128r32 r(id + 5000);
 
   for (size_t n = 0; n < swapcount; n++)