fix: random crash when starting or reloading

[dunglas]

Ugly fix for crashes reported in laravel/octane#764.

test-reload.sh is a reproducer. The crash produces the following stack trace (collected using lldb on Mac):

* thread #192, stop reason = EXC_BREAKPOINT (code=1, subcode=0x18d6df21c)
    frame #0: 0x000000018d6df21c libsystem_pthread.dylib`_pthread_mutex_firstfit_wake + 96
libsystem_pthread.dylib`:
->  0x18d6df21c <+96>: brk    #0x1

libsystem_pthread.dylib`:
    0x18d6df220 <+0>:  pacibsp 
    0x18d6df224 <+4>:  stp    x29, x30, [sp, #-0x10]!
    0x18d6df228 <+8>:  mov    x29, sp
Target 0: (frankenphp) stopped.
(lldb) bt
* thread #192, stop reason = EXC_BREAKPOINT (code=1, subcode=0x18d6df21c)
  * frame #0: 0x000000018d6df21c libsystem_pthread.dylib`_pthread_mutex_firstfit_wake + 96
    frame #1: 0x000000018d6dcdb4 libsystem_pthread.dylib`_pthread_mutex_firstfit_unlock_slow + 244
    frame #2: 0x0000000101232510 frankenphp`thpool_add_work + 184
    frame #3: 0x0000000101233984 frankenphp`manager_thread + 116
    frame #4: 0x000000018d6e2034 libsystem_pthread.dylib`_pthread_start + 136

It looks like there is a race condition during the startup of https://github.com/Pithikos/C-Thread-Pool, this sleep is a workaround to be sure that threads are ready before sending them work.

A better fix should be possible, but I don't understand why the threads aren't ready while there is code in the library for that.