SH-4139 Convert http downloaders and responders to llcorehttp patterns

Conversion was mostly trivial.  Did some refactoring in the conversion
of Responders to Handlers which eliminated 5X code replication.  More
will be done especially as this is extended to deal with the various
possible combinations of 200/206/416 status for ranged gets.  There are
a lot of thread races in the existing code, that is going to need some
real attention.  And the scheduling/liveness logic in the thread
management bounces around from thread to thread wasting a lot of time
and using expensive synchronization.  Much can be done here.  But the
result is that the 8 connections in the Mesh corehttp class now perform
as did the 32 connections of the original.  And that 32 actually looks
like it could bleed to over 64.  So, progress...