SwarmHandler: Defend against NPE when adding PEX seeds to a non-started client.
Use more NIO-like calls for storage.
Flush all FileChannel operations before close().
Make finish() idempotent, as it should be.
Abstract ByteStorage a little more.
Use voidPromise() and PooledByteBufAllocator in Netty for more performance.
Don't double-write blocks which were double-requested (still need to keep a map to PieceHandler(s) for sharing).