Fixed period AIS subscription loses updates

[vokkim]

Fix for issue: #220

Fixes subscription protocol when subscribing to AIS targets with fixed period:

{
  context: 'vessels.*',
  subscribe: [
    {path: '*', period: 10000}
  ]
}

Previous implementation lost vessel data while sampling the incoming events. This fix will buffer all incoming filteredBus events and ensure that one value for each vessel+source+path will be emitted after the given period.

Remaining issues:

• Several delta events will be emitted simultaneously on every period, each containing only one updates value. Optimal solution would emit only one delta event containing everything, but this would require some changes to the internal subscription architecture.
• minPeriod is left untouched and will still lose some AIS vessel data.