improve hp_rec_key_cmp string comparison

hp_rec_key_cmp performs two string comparison in two phases: while the first one it searches
an octet length of comparing strings and while the second one it does comparison using strnncollsp.
The main issue is that hp_rec_key_cmp decodes all the comparing strings completely whereas they could differ starting just from their first characters. Besides the fact that this issue has no performance impact in case of fixed length encoding, UTF-8 performance is suffering a lot.