func_odbc: single database connection should be optional

func_odbc was changed in Asterisk 13.9.0
to make func_odbc use a single database connection per DSN
because of reported bug ASTERISK-25938
with MySQL/MariaDB LAST_INSERT_ID().

This is drawback in performance when func_odbc is used
very often in dialplan.

Single database connection should be optional.

ASTERISK-26010

Change-Id: I7091783a7150252de8eeb455115bd00514dfe843