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.

changeset:   0:9c2cf2b35aa7
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     add foo

changeset:   1:3756a9556b89
tag:         tip
user:        test
date:        Mon Jan 12 13:46:41 1970 +0000
summary:     change foo

2 revisions:
9c2cf2b35aa7
3756a9556b89