[MEDIUM] add support for "balance hdr(name)"

[MEDIUM] add support for "balance hdr(name)"

There is a patch made by me that allow for balancing on any http header
field.

[WT:
  made minor changes:
  - turned 'balance header name' into 'balance hdr(name)' to match more
    closely the ACL syntax for easier future convergence
  - renamed the proxy structure fields header_* => hh_*
  - made it possible to use the domain name reduction to any header, not
    only "host" since it makes sense to do it with other ones.
  Otherwise patch looks good.
/WT]