PeerGroup: clear some static analysis warnings. Remove dead code.
Priority queue and exponential backoff for PeerGroup

Connect to peers in a service loop thread.

Resolves issue #159
Resolves issue #503
Cleanup.

Add lots more nullity annotations.
Clear lots of nullity static analysis warnings.
Delete some dead code.
Simplify a few expressions.

Resolves issue 317.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	core/src/main/java/com/google/bitcoin/net/discovery/SeedPeers.java
	core/src/main/java/com/google/dogecoin/core/PeerGroup.java
	core/src/main/java/com/google/dogecoin/core/Transaction.java
	core/src/main/java/com/google/dogecoin/protocols/channels/PaymentChannelServerListener.java
	core/src/test/java/com/google/dogecoin/core/PeerGroupTest.java
	core/src/test/java/com/google/dogecoin/core/TestWithPeerGroup.java
	examples/src/main/java/com/google/dogecoin/examples/PrintPeers.java