transport: add send operations to ClientStream and ServerStream

[dfawley]

More background refactoring work to potentially allow us to create an interceptor that can read raw bytes, and a couple very minor cleanups, too.

Next up I'm thinking of changing ReadMessageHeader(buf) (I found ReadHeader ambiguous and confusing) and Read(n) into a single Read() (isCompressed bool, msgBytes mem.BufferSlice, err error). This means the transport needs to understand the grpc header and max message size configuration instead of the grpc layer, but that actually seems pretty straightforward and also appropriate.

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 96.52174% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.82%. Comparing base (2a18bfc) to head (7c61aec).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
server.go	89.47%	0 Missing and 2 partials ⚠️
stream.go	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7808      +/-   ##
==========================================
- Coverage   81.88%   81.82%   -0.06%     
==========================================
  Files         373      373              
  Lines       37700    37698       -2     
==========================================
- Hits        30870    30847      -23     
- Misses       5547     5567      +20     
- Partials     1283     1284       +1     

Files with missing lines	Coverage Δ
internal/transport/client_stream.go	100.00% <100.00%> (ø)
internal/transport/handler_server.go	86.46% <100.00%> (-0.05%)	⬇️
internal/transport/http2_client.go	91.51% <100.00%> (-0.34%)	⬇️
internal/transport/http2_server.go	91.00% <100.00%> (+0.59%)	⬆️
internal/transport/server_stream.go	95.31% <100.00%> (+6.42%)	⬆️
internal/transport/transport.go	84.73% <100.00%> (-6.83%)	⬇️
rpc_util.go	79.37% <100.00%> (-0.17%)	⬇️
server.go	81.98% <89.47%> (-0.89%)	⬇️
stream.go	81.86% <80.00%> (+0.14%)	⬆️

... and 18 files with indirect coverage changes