allow to configure the maximum effective interval between attempts to read data by plugins
rename option MaxEffectiveInterval -> MaxReadInterval
document MaxReadInterval option
parse option MaxReadInterval in the plugin_init_all()
src/configfile.c: Use cf_util_get_cdtime() to parse the "Interval" settings.
src/configfile.[ch]: Implement global_option_get_time().
src/plugin.c: Parse "MaxReadInterval" with global_option_get_time().
Merge branch 'pr-713'