add silence period a.k.a. debounce

[pavel-kolesnikov]

This will add a per-block defined silence period (widely known as debounce).
Hopefully will suffice to close #86 (also close #55).

Default behavior: no pause.

Example:

** {
  silence: 1s
  prep: echo "no more then once a second"
}

Will produce some backoff log like this

Also I used Note log level, but had no idea how to make the message from debouncer to be not visible by default. Help hre appreciated.