WL#2935: MySQL plugin interface: status variables

Plugin should be able to specify status variables that are shown in SHOW STATUS

st_plugin has a field for an array of variables, when plugin is installed, this
array is added to the list of variables in the pluginname_ namespace (see how
innodb status variables are added now)

st_plugin also has a field for a function that is called before these variables
are accessed.

even better would be to add newe status variable type - a function.
This function returns variable type and variable data.
this way we eliminate innodb_update_status, and plugin's function mentioned above.
instead innodb_update_status goes directly into status variable array and
returns variable array that is stored there now.

Having this function status type, we should remove all "specific" status types,
like SHOW_UPTIME, etc. and only keep generic SHOW_STR, SHOW_LONG, and so on.
show status types should describe the representation of the data, not the semantics.