Make zlib encoding messages idempotent.

Otherwise duplicate messages can trigger a reinitialization of the
compression stream while the update thread is running. Also ensure
that the stream is initialized before the update thread may attempt
to use it.

PR: 238333
Reviewed by: cem, rgrimes
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20673