Replacing the forked children with a gather kept their fan-out: every
row of the import file gets a session of its own and they all start at
once, so a large file opens a local socket and a server side session
task per node simultaneously.
Hold a semaphore for the duration of each node's assignment instead, so
a finished node's session is dropped before the next one starts. Also
build that session once per node rather than once per MAC, and say why
the caller's session is not reused, which was self evident while this
ran in a forked child.
Markus Hilger
a619b6ed6f