Rename Socket::read to Socket::recv.

This is more consistent: send/recv vs. send/read.

Review: https://reviews.apache.org/r/28484
Renamed poll.cpp to libev_poll.cpp.

Review: https://reviews.apache.org/r/28314
Extended install() to support more arguments.

Review: https://reviews.apache.org/r/28363
Refactored task launching in the master.

This adds addTask(), which is similar to addSlave() and addFramework().
The benefit is that we allow the caller to add the task and have control
over when the RunTaskMessage is sent.

Review: https://reviews.apache.org/r/28364