idoutils: allow to disable customvar status updates with dump_customvar_status=0|1 in idomod.cfg

this commit also contains a fix for not making a difference between
customvariable config and status data buffers.

the new default is disabling customvar status updates (if config option
is not set), as in most scenarios these updates/table remains useless
afterall. the customvar status updates are forcibly triggered on *every*
host/service (check) update and may decrease rdbms performance in large
scale systems having lots of custom variables applied (i.e. for
filters/restrictions in icinga web).

refs #4479