lei: consolidate the bulk of the IPC code

lei: consolidate the bulk of the IPC code

The backends for "lei add-external --mirror", "lei convert", and
"lei import" all share a similar pattern for spawning background
workers.  Hoist out the common parts to slim down our code base
a bit.

The LeiXSearch and LeiToMail workers for "lei q" remains a the
odd duck due to the deep pipelining and parallelization.