MINOR: server: change adm_st_chg_cause storage type

MINOR: server: change adm_st_chg_cause storage type

Even though it doesn't look like it at first glance, this is more like
a cleanup than an actual code improvement:

Given that srv->adm_st_chg_cause has been used to exclusively store
static strings ever since it was implemented, we make the choice to
store it as an enum instead of a fixed-size string within server
struct.

This will allow to save some space in server struct, and will make
it more easily exportable (ie: event handlers) because of the
reduced memory footprint during handling and the ability to later get
the corresponding human-readable message when it's explicitly needed.