timer: Add immediate option to timer_execute

[repeatedly]

Enhancement request for input plugins.
By default, PluginHelper::Timer#timer_execute launches timer immediately.
This is no problem on almost cases but some plugins, e.g. in_tail, takes long time in start by executing code.
To avoid this problem, delaying timer execution mitigates this situation.

See also: #1502

[fujimotos]

FWIW, I also want to use this option in Windows exporter.

• We use timer_execute to periodically collect system metrics (default interval is 60s).
• I feel the current user experience is sub-optimal, because our user needs
  to wait 1 minute to confirm that the plugin is actually working.

I was considering to migrate to manual timer implementation because of this issue,
but it is preferable if timer_execute supports immediate option.