Implement pgcopydb stream prefetch. (#65)

* Implement pgcopydb stream prefetch.

The prefetch command setup the logical decoding operations in a way that
allows transforming each JSON file received into an SQL file. It's a kind of
integration of the two following commands:

    $ pgcopydb stream receive
    $ pgcopydb stream transform

Where the transform process is started in the background as soon as needed.

* Implement pgcopydb stream setup.

Also implement two other new commands to create and drop a replication
origin, so that the command

    $ pgcopydb stream setup

is equivalent to the commands

    $ pgcopydb create slot
    $ pgcopydb create origin --startpos <slot creation lsn>