www: wire up search to use async xap_helper
The C++ version of xap_helper will allow more complex and
expensive queries. Both the Perl and C++-only version will
allow offloading search into a separate process which can be
killed via ITIMER_REAL or RLIMIT_CPU in the face of overload.
The xap_helper `mset' command wrapper is simplified to
unconditionally return rank, percentage, and estimated matches
information. This may slightly penalize mbox retrievals and
lei users, but perhaps that can be a different command entirely.
14 files changed: