Rename bbstreamer to astreamer.

Rename bbstreamer to astreamer.

I (rhaas) intended "bbstreamer" to stand for "base backup streamer,"
but that implies that this infrastructure can only ever be used by
pg_basebackup.  In fact, it is a generally useful way of streaming
data from a tar or compressed tar file, and it could be extended to
work with other archive formats as well if we ever wanted to do that.
Hence, rename it to "astreamer" (archive streamer) in preparation for
reusing the infrastructure from pg_verifybackup (and perhaps
eventually also other utilities, such as pg_combinebackup or
pg_waldump).

This is purely a renaming commit. Comment adjustments and relocation
of the actual code to someplace from which it can be reused are left
to future commits.

Amul Sul, reviewed by Sravan Kumar and by me.

Discussion: http://postgr.es/m/CAAJ_b94StvLWrc_p4q-f7n3OPfr6GhL8_XuAg2aAaYZp1tF-nw@mail.gmail.com