hgweb: split "verbs" into methods.

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.

http server: support persistent connections. only "hg serve" affected yet. http server running cgi script will not use persistent connections. support for fastcgi will help that. clients that support keepalive can use one tcp connection for all commands during clone and pull. this makes latency of binary search during pull much lower over wan. if server does not know content-length, it will force connection to close at end. right fix is to use chunked transfer-encoding but this is easier and does not hurt performance. only command that is affected is "changegroup" which is always last command during a pull.