Update ADOdb to latest 5.19dev

Using development release as it contains fixes over official 5.18 which
are required for MantisBT to work properly, and John Lim (ADOdb author)
is not responding to repeated requests to release 5.19.
Added constant for ADOdb minimum version

Modified installer and admin checks page to use the new constant
Fix admin checks for mysqli table and field collation

The mysqli driver returns field names in lowercase for SHOW TABLE STATUS
whereas mysql uses an uppercase first letter.

Fixes #16968
Install: fix variable names in generated config file

The config_inc.php file generated by the installer had incorrect
variable names for the table prefix and suffix, missing the 'g_'.

Fixes #16966
Admin checks: added test for db_table_plugin_prefix

We now display the current value (like we do for the other 2 variables),
and display a warning when it is empty.

Fixes #16969