MDEV-20279 Increase Aria index length limit

Limit increased from 1000 to 2000.

Avoiding stack overflow by only storing keys and pages on the stack in
recursive functions if there is plenty of space on it.

Other things:
- Use less stack space for b-tree operations as we now only allocate as
  much space as needed instead of always allocating HA_MAX_KEY_LENGTH.
- Replaced most usage of my_safe_alloca() in Aria with the stack_alloc
  interface.
- Moved my_setstacksize() to mysys/my_pthread.c

include/my_pthread.h

@@ -312,6 +312,8 @@ int my_pthread_mutex_trylock(pthread_mutex_t *mutex);
 #endif
 #endif
 
+size_t my_setstacksize(pthread_attr_t *attr, size_t stacksize);
+
 /*
   The defines set_timespec and set_timespec_nsec should be used
   for calculating an absolute time at which