- Add shutdown() method (to be used in forthcoming call combiner code).
- Use a vtable instead of storing method pointers in each instance.
- Check all callers of pull() to make sure that they are properly
  handling errors.
- Clarify ownership rules and attempt to adhere to them.
- Added a new grpc_caching_byte_stream implementation, which is used in
  http_client_filter to avoid having to read the whole send_message byte
  stream before passing control down the stack.  (This class may also be
  used in the retry code I'm working on separately.)
- As part of this, did a major rewrite of http_client_filter, which
  made the code more readable and fixed a number of potential bugs.

Note that some of this code is hard to test right now, due to the fact
that the send_message byte stream is always a slice_buffer stream, for
which next() is always synchronous and no destruction is needed.
However, some future work (specifically, my call combiner work and
Craig's incremental send work) will start leveraging this.