compress/flate: implement Flush

compress/flate: implement Flush

This Flush is equivalent to zlib's Z_SYNC_FLUSH.
The addition of the explicit Writer type opens the
door to adding a PartialFlush if needed for SSH
and maybe even FullFlush.  It also opens the door
for a SetDictionary method to be added.

http://www.bolet.org/~pornin/deflate-flush.html
documents the various intricacies of flushing a
DEFLATE stream.

R=agl1, r
CC=golang-dev
https://golang.org/cl/3637041