Don't construct unnecessary buffers.

[Lukasa]

Motivation:

Currently the HTTP1ToGRPCServerCodec constructs a CircularBuffer that it
only ever uses when doing grpc-web work. This is not the end of the
world, except that CircularBuffers require memory allocations: there is
no such thing as an empty CircularBuffer.

In general it's not a great idea to force the mainline grpc use-case to
pay for the cost of constructing a circular buffer it will never use, so
instead we replace this with an optional circular buffer. That optional
will be initialized on first use, so it doesn't add any complexity to
our state tracking. It does add some subtle complexity to the rest of
the grpc-web code, though, so we should consider increasing the priority
of dealing with grpc-web properly.

Modifications:

• Make HTTP1ToGRPCServerCodec.responseTextBuffers optional, delay its
  initialization.

Results:

Small performance improvement on HTTP/2 server benchmarks, around 0.7%
per RPC.

[glbrntt]

Ace.