Bug #1921: ftp.cc(3309) ICAP cannot keep up with FTP; lost 1448/1448 bytes.

Bug #1921: ftp.cc(3309) ICAP cannot keep up with FTP; lost 1448/1448 bytes.

This patch adds a little buffering before the BodyPipe, allowing more
data to be temporarily stuffed than what fits in the pipe. Needed for
FTP directory listings and other sources with no byte-exact flow control.

The patch also moves most ICAP dependencies from the protocol implementations
to the shared ServerStateData parent, allowing them all to share logics.