Turned pconn.cc into C++ classes. There are now separate persistent

Turned pconn.cc into C++ classes.  There are now separate persistent
connection pools for client-side, server-side, and now ICAP.

Accounting for persistent connection use still sucks because HTTP
(http.c) and ICAP state structures don't have any way to keep track
of how many times a given TCP connection was used.  So we'll keep
abusing fd_entry[] for this purpose.