lei q: skip lei/store->write_prepare for JSON outputs

lei q: skip lei/store->write_prepare for JSON outputs

JSON outputs won't write to lei/store at all, so there's
no point in forking the store worker if it's not already
running.

LeiSearch object ($lse) is also fork-safe until it opens a
persistent FD for Xapian/SQLite so we can unconditionally
carry it across fork.