Fix background thread initialization race

[puzpuzpuz]

The race condition happens when per-cpu arenas and background threads features are enabled:

1. Thread A calls malloc() first -> starts malloc_init_hard()
2. While Thread A is still in malloc_init_hard(), Threads B, C, D also call malloc()
3. These threads trigger arena creation (with percpu_arena, each CPU gets its own arena)
4. arena_init() -> arena_new_create_background_thread() -> background_thread_create(arena_ind=12, etc.)
5. Thread A hasn't reached line 2232 (background_thread_create(tsd, 0)) yet
6. So a non-zero arena can mark thread slot 0 as "started" before arena 0 does

The initialization has locks, but arena creation for different arenas can proceed in parallel as soon as malloc_init_state = initialized and init_lock is released, and the background_thread_create(tsd, 0) call happens late in malloc_init_hard() (after arenas are already being created by other threads).

This bug won't be noticeable for applications that have single-threaded initialization path, but in case of JVM I observed the following threads allocating concurrently at startup:

Thread	Description
java	Main JVM thread
C1 CompilerThre	JIT C1 compiler thread
JVMCI-native Co	GraalVM JVMCI compiler thread
GC Thread#5, 10, 15	G1 garbage collector threads
shared-network_	QuestDB network worker threads
Thread-1	Generic Java thread

Reproducer

Can be reproduced with QuestDB's rt binary on Linux and the following script:

#!/bin/bash

cd <project_dir>/target/questdb-9.2.4-SNAPSHOT-rt-linux-x86-64

for i in {1..50}; do
  echo "=== Run $i ==="
  ./bin/questdb.sh start -d <qdb_root>
  sleep 2
  pid=$(pgrep -f QuestDB-Runtime)
  if [ -n "$pid" ]; then
    count=$(sudo gdb -p $pid -batch -ex "info threads" 2>/dev/null | grep -c jemalloc_bg_thd)
    echo "jemalloc_bg_thd threads: $count"
  else
    echo "QuestDB not running"
  fi
  ./bin/questdb.sh stop
  sleep 1
done

When jemalloc_bg_thd threads: 0 gets printed, it means that jemalloc's background threads didn't start due to the race.

[puzpuzpuz]

To run tests:

./autogen.sh
make
make check