[Feature] Add task registry for safe Lua task reference validation

[Feature] Add task registry for safe Lua task reference validation

Implement a global task registry that maps unique uint64_t keys to task
pointers. This prevents use-after-free bugs when Lua code holds references
to tasks that may have been freed (e.g., in async Redis callbacks).

Key changes:
- Add lua_key field to rspamd_task struct
- Implement task registry using khash (O(1) lookup)
- Store lua_key in Lua userdata instead of raw pointer
- Lookup via registry when extracting task from Lua
- Remove task from registry FIRST in rspamd_task_free()

The counter-based key approach avoids issues with:
- Pointer reuse after free (memory allocator may reuse addresses)
- Lua number precision (52-bit mantissa is sufficient for counter)
- NaN/subnormal float values that could cause issues

This fixes potential use-after-free in Redis script waitq callbacks
when Redis is unavailable longer than task lifetime.