Added 'hostname' flag to the master.

Review: https://reviews.apache.org/r/17210
Added 'protobuf::createMasterInfo()' to create MasterInfo from UPID.

Review: https://reviews.apache.org/r/17572
Changed master and slave to get the default hostname based on the ip.

Review: https://reviews.apache.org/r/17573
Updated detector to return MasterInfo instead of PID.

Review: https://reviews.apache.org/r/17574
Updated master redirection to use leading master's hostname.

Review: https://reviews.apache.org/r/17575