daemon: Task abstraction + TaskStatus
daemon: Task abstraction + TaskStatus

An instance of Task tracks a single thread of activity that is part of a Job.

While the docs already use this terminology of tasks being composed of jobs,
the code did not have an object to represent these semantics.
Now it does:

* A task t is initialized with a root activity, which is its name
* t can t.Enter() and t.Finish() an activity, building
  a stack of activities
* t's code can get a logger t.Log() whose logTaskField is set to the
  concatenated stack of activities
* t's code can update IO progress it made since leaving idle state
* t's code's log output vie t.Log() is captured since leaving idle
  state
  * FIXME: find a way to bound that buffer

refs #10
refs #48

zrepl/zrepl,zrepl/zrepl