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.
(cherry picked from commit 683dfaa63df0af317f7485042539bdacf8646eab)