SH-4371 Reduce 22mS inter-connection latency.

This really extended into the client-side request throttling.
Moved this from llmeshrepository (which doesn't really want
to do connection management) into llcorehttp.  It's now a
class option with configurable rate.  This still isn't the
right thing to do as it creates coupling between viewer
and services.  When we get to pipelining, this notion becomes
invalid.