moved method computation in a if block only when send_initial_metadata
is true.

modified comment about when GET verb is used.
Added code to read data from send_message and defer the op when
  it is not fully available.
clang-format one more time.

Change names to StreamedUnary, ServerUnaryStreamer, etc. Use a templated method handler since most code shared between the new StreamedUnary and the existing BidiStreaming. Eliminate the separate enum case for streamed unary. Return a status failure if a StreamedUnary method handler doesn't actually do a write (since that is
violating the appearance of unary-ness)

freeing up payload_bytes.
returning GET response only once.

The HTTP/2 specification requires header names to be lower case.