Optimize the use of hex2utf8

[1Hyena]

As it turns out, with longer payloads in OP_RETURN it can happen that hex2utf8 Lua function becomes vulnerable to Denial-of-Service attacks. For example, TX dc5d9c5c98a11bb404b9c139a377a1326b27a25c6750059f16076519d65709a2 in block 566530 is one such problematic transaction. First of all, the utf8 extraction should be implemented in C++ rather than in Lua. Secondly, such extractions should be executed in parallel.