Bugfix: The global s_get_timestamp() function is returning current time instead of...

Bugfix: The global s_get_timestamp() function is returning current time instead of requested time

It looks like I copied the s_get_date() function and forgot to properly updated it (or got confused and forgot what I was working on).
Either way, the implementation is incorrect and does not produce the correct timestamp.

The function is rewritten to only support a time string to be formatted into a unix timestamp float.

The base_defaults_global implementation is also not consistent with the one in reservation_defaults_global.
The base_defaults_global is intended to be a default example implementation where reservation_defaults_global is an implementation specific to the reservation system.

The postgresql default datetime string format is provided as a global constant in the database class for consistency.