Improve performance

[mucaho]

As this library is also intended to work on Android, performance should be optimized where applicable

• send only 1 packet (no duplicate ack/last ack fields) for multiple for retransmits
  -> add Controller.send(Collection<MultiKeyValue>): Packet
  -> add Controller.receive(Packet with Collection<MultiKeyValue>): Object
• make sure that there is minimal object allocation inside scopes -> extract such local variables as fields
• add object pools for Packet, MultiKeyValue, Iterator and co instead of instantiating new objects
  -> update serializer object creation strategy to use object pools
• use practical initialCapacity parameters during collection initialization
• use a rolling fixed size queue for pending acks, eg. a 256 size queue indexed by sequence number mod 256. changing to plain array much of the NavigableMap functionality is lost -> rather investigate alternative collection libraries with near zero garbage