Choosing target DataNodes

[nichhk]

The NameNode needs to choose target DataNodes for a block using 3 criteria:

1. current DataNode locations for the block, if it is not new
2. the free disk space on a DataNode (getFreeSpace & getTotalSpace methods in data persistence layer #58)
3. the number of transmissions on a DataNode (Maintain number of transmissions in data transfer server #59)

[nichhk]

@kyler-m: choose a random subset of valid DataNodes.

[prb2]

Implemented find_datanode_for_block to use free_bytes and xmits in: 54f8df3

Live DNs which do not already have a replica of the requested block and have
enough free space to hold the block are pushed onto a priority queue based on
num transmits. Then, the number of reqeusted datanodes (replication_factor
many) are popped from the priority queue and returned to the caller.

Some clean up work remains.

[prb2]

You can see in the image that we find two DNs that are:

1. alive
2. do not already contain the block which needs to be replicated
3. have enough free space.

The first one we see has 5 transmits, the next one has 3 transmits. Since we minimize transmits, the one with 3 transmits is selected and returned.