http client: support persistent connections. uses keepalive module from urlgrabber package. tested against "hg serve", cgi server, and through http proxy. used ethereal to verify that only one tcp connection used during entire "hg pull" sequence. if server supports keepalive, this makes latency of "hg pull" much lower.

port:4:export
port:4:vaportight
port:4:import/export
FIXME: history is wrong here
port:1:1:-:eggs:import
port:1:2:+:eggs:vaportight
port:1:3:+:eggs:import/export
port:0:2:+:spam:export
port:0:1:+:spam:import
port:4:import/export