db_unixodbc: fix query result handling and improve performance

Until now, query rows were incrementally stored in a list with
separately allocated length and string buffers, along with a great deal
of useless pointer dereferencing, effectively wasting CPU cycles and pkg
mem usage.

This patch eliminates the linked list logic with a global buffer of
pointers (doubling in size when required) meant to hold each result column.
Each column is transparently freed by the DB backend. This completes
commit d63ee19a8.