DefaultPeerStorage::addPeer: Remove unnecessary '='
Code cleanup
Refactor peer list management in DefaultPeerStorage

Peer list is now divided into 2: unusedPeers_ and usedPeers_.
Duplicate check is done using std::set by comparing pair of IP address
and port. For this, only IP address and port given to the Peer
constructor are used. In other words, TCP port received from extended
message is not used for this purpose.
Remove PeerStorage::getActivePeers() and add getUsedPeers() instead

PeerStorage::getUsedPeers() returns non-active peers, so caller must
call Peer::isActive() to get active peers.
DefaultPeerStorage::addBadPeer: Updating eoi is not required after erasure
Rename /BadCondition/ in Peer to /Drop/
Make DefaultPeerStorage::droppedPeers_ distinct