lei/store: use SOCK_SEQPACKET rather than pipe

lei/store: use SOCK_SEQPACKET rather than pipe

This has several advantages:

* no need to use ipc.lock to protect a pipe for non-atomic writes

* ability to pass FDs.  In another commit, this will let us
  simplify lei->sto_done_request and pass newly-created
  sockets to lei/store directly.

disadvantages:

- an extra pipe is required for rare messages over several
  hundred KB, this is probably a non-issue, though

The performance delta is unknown, but I expect shards
(which remain pipes) to be the primary bottleneck IPC-wise
for lei/store.