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/maria.h

@@ -186,6 +186,7 @@ typedef struct st_maria_keydef          /* Key definition with open & info */
   uint16 keylength;                     /* Tot length of keyparts (auto) */
   uint16 minlength;                     /* min length of (packed) key (auto) */
   uint16 maxlength;                     /* max length of (packed) key (auto) */
+  uint16 max_store_length;              /* Size to store key + overhead */
   uint32 write_comp_flag;		/* compare flag for write key (auto) */
   uint32 version;                       /* For concurrent read/write */
   uint32 ftkey_nr;                      /* full-text index number */