TS-1911: make MIOBufferAccessor members private

We already have sensibly-named accessors for the MIOBufferAccessor
members, so there's no need to access the internals directly. Make
the members private and use the accessors wherever necessary. This
exposes some places that don't use the proper APIs, so fix those
too.
TS-1912: SSL hangs after origin handshake

When using SSL with a Netscaler origin, I'm seeing the handshake
complete but ATS never issuing the HTTP request. It seems that while
the handshake is in progress, the write to the origin is not
rescheduled, so ATS never notices that it can send the origin
request.